Remove unneeded nbtree array preprocessing assert.

Certain cases involving the use of cursors had assertion failures within
_bt_preprocess_keys's recently added no-op return path.  The assertion
in question made the faulty assumption that a second or third call to
_bt_preprocess_keys (within the same btrescan) could only happen when
another scheduled primitive index scan was just about to begin.

It would be possible to address the problem by only allowing scans that
have array keys to take the new no-op path, forcing affected cases to
perform redundant preprocessing work.  It seems simpler to just remove
the assertion, and reframe the no-op path as a more general mechanism.
Take this simpler approach.

The important underlying principle is that we only need to perform
preprocessing once per btrescan (at most).  This is expected regardless
of whether or not the scan happens to have array keys.

Oversight in commit 1b134ca5, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/ef0f7c8b-a6fa-362e-6fd6-054950f947ca@gmail.com

src/backend/access/nbtree/nbtutils.c

@@ -2572,19 +2572,16 @@ _bt_preprocess_keys(IndexScanDesc scan)
 	int		   *keyDataMap = NULL;
 	int			arrayidx = 0;
 
-	/*
-	 * We're called at the start of each primitive index scan during scans
-	 * that use equality array keys.  We can just reuse the scan keys that
-	 * were output at the start of the scan's first primitive index scan.
-	 */
 	if (so->numberOfKeys > 0)
 	{
 		/*
-		 * An earlier call to _bt_advance_array_keys already set everything up
-		 * already.  Just assert that the scan's existing output scan keys are
-		 * consistent with its current array elements.
+		 * Only need to do preprocessing once per btrescan, at most.  All
+		 * calls after the first are handled as no-ops.
+		 *
+		 * If there are array scan keys in so->keyData[], then the now-current
+		 * array elements must already be present in each array's scan key.
+		 * Verify that that happened using an assertion.
 		 */
-		Assert(so->numArrayKeys);
 		Assert(_bt_verify_keys_with_arraykeys(scan));
 		return;
 	}