Remove overzealous array element type assertion.

This led to spurious assertion failures in certain scenarios involving
pseudo types.

Oversight in commit 5bf748b, which enhanced nbtree ScalarArrayOp
execution.

Reported-By: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs48f5rDOwxaT76Zd40m7n9iGZQcjEk7vG_5p3YWNh6oPfA@mail.gmail.com

Author: petergeoghegan

src/backend/access/nbtree/nbtutils.c

@@ -392,7 +392,6 @@ _bt_preprocess_array_keys(IndexScanDesc scan)
 		elemtype = cur->sk_subtype;
 		if (elemtype == InvalidOid)
 			elemtype = rel->rd_opcintype[cur->sk_attno - 1];
-		Assert(elemtype == ARR_ELEMTYPE(arrayval));
 
 		/*
 		 * If the comparison operator is not equality, then the array qual