Remove unneeded VM pin from VM replay

Previously, heap_xlog_visible() called visibilitymap_pin() even after
getting a buffer from XLogReadBufferForRedoExtended() -- which returns a
pinned buffer containing the specified block of the visibility map.

This would just have resulted in visibilitymap_pin() returning early
since the specified page was already present and pinned, but it was
confusing extraneous code, so remove it. It doesn't seem worth
backporting, though.

It appears to be an oversight in 2c03216.

While we are at it, remove two VM-related redundant asserts in the COPY
FREEZE code path. visibilitymap_set() already asserts that
PD_ALL_VISIBLE is set on the heap page and checks that the vmbuffer
contains the bits corresponding to the specified heap block, so callers
do not also need to check this.

Author: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Reported-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>

Discussion: https://postgr.es/m/CALdSSPhu7WZd%2BEfQDha1nz%3DDC93OtY1%3DUFEdWwSZsASka_2eRQ%40mail.gmail.com

src/backend/access/heap/heapam.c

@@ -2647,9 +2647,6 @@ heap_multi_insert(Relation relation, TupleTableSlot **slots, int ntuples,
 		 */
 		if (all_frozen_set)
 		{
-			Assert(PageIsAllVisible(page));
-			Assert(visibilitymap_pin_ok(BufferGetBlockNumber(buffer), vmbuffer));
-
 			/*
 			 * It's fine to use InvalidTransactionId here - this is only used
 			 * when HEAP_INSERT_FROZEN is specified, which intentionally

src/backend/access/heap/heapam_xlog.c

@@ -295,7 +295,6 @@ heap_xlog_visible(XLogReaderState *record)
 		LockBuffer(vmbuffer, BUFFER_LOCK_UNLOCK);
 
 		reln = CreateFakeRelcacheEntry(rlocator);
-		visibilitymap_pin(reln, blkno, &vmbuffer);
 
 		visibilitymap_set(reln, blkno, InvalidBuffer, lsn, vmbuffer,
 						  xlrec->snapshotConflictHorizon, vmbits);