Fix hot standby bug with GiST scans.

Don't reset the rightlink of a page when replaying a page update record.
This was a leftover from pre-hot standby days, when it was not possible to
have scans concurrent with WAL replay. Resetting the right-link was not
necessary back then either, but it was done for the sake of tidiness. But
with hot standby, it's wrong, because a concurrent scan might still need it.

Backpatch all versions with hot standby, 9.0 and above.

src/backend/access/gist/gistxlog.c

@@ -253,7 +253,6 @@ gistRedoPageUpdateRecord(XLogRecPtr lsn, XLogRecord *record, bool isnewroot)
 		 */
 		GistPageSetLeaf(page);
 
-	GistPageGetOpaque(page)->rightlink = InvalidBlockNumber;
 	PageSetLSN(page, lsn);
 	PageSetTLI(page, ThisTimeLineID);
 	MarkBufferDirty(buffer);