Remove extra nbtree half-dead internal page check.

It's not safe for nbtree VACUUM to attempt to delete a target page whose
right sibling is already half-dead, since that would fail the
cross-check when VACUUM attempts to re-find a downlink to the right
sibling in the parent page.  Logic to prevent this from happening was
added by commit 8da3183780, which addressed a bug in the overhaul of
page deletion that went into PostgreSQL 9.4 (commit efada2b8e9).
VACUUM was made to check the right sibling page, and back off when it
happened to be half-dead already.

However, it is only truly necessary to do the right sibling check on the
leaf level, since that transitively determines if the deletion target's
parent's right sibling page is itself undergoing deletion.  Remove the
internal page level check, and add a comment explaining why the leaf
level check alone suffices.

The extra check is also unnecessary due to the fact that internal pages
that are marked half-dead are generally considered corrupt.  Commit
efada2b8e9 established the principle that there should never be
half-dead internal pages (internal pages pending deletion are possible,
but that status is never directly represented in the internal page).
VACUUM will complain about corruption when it encounters half-dead
internal pages, so VACUUM is bound to raise an error one way or another
when an nbtree index has a half-dead internal page (contrib/amcheck will
also report that the page is corrupt).

It's possible that a pg_upgrade'd 9.3 database will still have half-dead
internal pages, so it may seem like there is an argument for leaving the
check in place to reliably get a cleaner error message that advises the
user to REINDEX.  However, leaf pages are also deleted in the first
phase of deletion prior to PostgreSQL 9.4, so I believe we won't even
attempt to re-find the parent page anyway (we won't have the fully
deleted leaf page as the right sibling of our target page, so we won't
even try to find a downlink for it).

Discussion: https://postgr.es/m/CAH2-Wzm_ntmqJjWLRyKzimFmFvk+BnVAvUpaA4s1h9Ja58woaQ@mail.gmail.com

src/backend/access/nbtree/README

@@ -262,11 +262,15 @@ long as we're keeping the leaf locked.  The internal pages in the chain
 cannot acquire new children afterwards either, because the leaf page is
 marked as half-dead and won't be split.
 
-Removing the downlink to the top of the to-be-deleted chain effectively
-transfers the key space to the right sibling for all the intermediate levels
-too, in one atomic operation.  A concurrent search might still visit the
-intermediate pages, but it will move right when it reaches the half-dead page
-at the leaf level.
+Removing the downlink to the top of the to-be-deleted subtree/chain
+effectively transfers the key space to the right sibling for all the
+intermediate levels too, in one atomic operation.  A concurrent search might
+still visit the intermediate pages, but it will move right when it reaches
+the half-dead page at the leaf level.  In particular, the search will move to
+the subtree to the right of the half-dead leaf page/to-be-deleted subtree,
+since the half-dead leaf page's right sibling must be a "cousin" page, not a
+"true" sibling page (or a second cousin page when the to-be-deleted chain
+starts at leaf page's grandparent page, and so on).
 
 In the second stage, the topmost page in the chain is unlinked from its
 siblings, and the half-dead leaf page is updated to point to the next page

src/backend/access/nbtree/nbtpage.c

@@ -1301,17 +1301,6 @@ _bt_lock_branch_parent(Relation rel, BlockNumber child, BTStack stack,
 				_bt_relbuf(rel, lbuf);
 			}
 
-			/*
-			 * Perform the same check on this internal level that
-			 * _bt_mark_page_halfdead performed on the leaf level.
-			 */
-			if (_bt_is_page_halfdead(rel, *rightsib))
-			{
-				elog(DEBUG1, "could not delete page %u because its right sibling %u is half-dead",
-					 parent, *rightsib);
-				return false;
-			}
-
 			return _bt_lock_branch_parent(rel, parent, stack->bts_parent,
 										  topparent, topoff, target, rightsib);
 		}
@@ -1621,7 +1610,17 @@ _bt_mark_page_halfdead(Relation rel, Buffer leafbuf, BTStack stack)
 	 * parent, unless it is the only child --- in which case the parent has to
 	 * be deleted too, and the same condition applies recursively to it. We
 	 * have to check this condition all the way up before trying to delete,
-	 * and lock the final parent of the to-be-deleted branch.
+	 * and lock the final parent of the to-be-deleted subtree.
+	 *
+	 * However, we won't need to repeat the above _bt_is_page_halfdead() check
+	 * for parent/ancestor pages because of the rightmost restriction. The
+	 * leaf check will apply to a right "cousin" leaf page rather than a
+	 * simple right sibling leaf page in cases where we actually go on to
+	 * perform internal page deletion. The right cousin leaf page is
+	 * representative of the left edge of the subtree to the right of the
+	 * to-be-deleted subtree as a whole.  (Besides, internal pages are never
+	 * marked half-dead, so it isn't even possible to directly assess if an
+	 * internal page is part of some other to-be-deleted subtree.)
 	 */
 	rightsib = leafrightsib;
 	target = leafblkno;