Remove obsolete nbtree page deletion comment.

Commit efada2b8e9, which made the nbtree page deletion algorithm more
robust, removed the concept of a half-dead internal page.  Remove a
comment about half dead parent pages that was overlooked.

src/backend/access/nbtree/nbtpage.c

@@ -1930,9 +1930,6 @@ _bt_unlink_halfdead_page(Relation rel, Buffer leafbuf, bool *rightsib_empty)
 	 * might be possible to push the fast root even further down, but the odds
 	 * of doing so are slim, and the locking considerations daunting.)
 	 *
-	 * We don't support handling this in the case where the parent is becoming
-	 * half-dead, even though it theoretically could occur.
-	 *
 	 * We can safely acquire a lock on the metapage here --- see comments for
 	 * _bt_newroot().
 	 */

src/backend/access/nbtree/nbtutils.c

@@ -2388,7 +2388,7 @@ _bt_check_natts(Relation rel, bool heapkeyspace, Page page, OffsetNumber offnum)
 	int			tupnatts;
 
 	/*
-	 * We cannot reliably test a deleted or half-deleted page, since they have
+	 * We cannot reliably test a deleted or half-dead page, since they have
 	 * dummy high keys
 	 */
 	if (P_IGNORE(opaque))