Remove useless pointer update in ginxlog.c

Oversight in 2c03216d83, when the redo code of GIN got refactored for
the new WAL format where block information has been standardized, as the
payload data got tracked for each block after the change, and not in the
whole record.  This is just a cleanup.

Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/CALdSSPgnAt5L=D_xGXRXLYO5FK1H31_eYEESxdU1n-r4g+6GqA@mail.gmail.com

src/backend/access/gin/ginxlog.c

@@ -368,7 +368,6 @@ ginRedoInsert(XLogReaderState *record)
 #endif
 		payload += sizeof(BlockIdData);
 		rightChildBlkno = BlockIdGetBlockNumber((BlockId) payload);
-		payload += sizeof(BlockIdData);
 
 		ginRedoClearIncompleteSplit(record, 1);
 	}