Remove duplicate include of slot.h.

Back-patch to 9.4, where this problem was added.

Author: MasaoFujii

src/backend/replication/walsender.c

@@ -62,7 +62,6 @@
 #include "replication/slot.h"
 #include "replication/snapbuild.h"
 #include "replication/syncrep.h"
-#include "replication/slot.h"
 #include "replication/walreceiver.h"
 #include "replication/walsender.h"
 #include "replication/walsender_private.h"