Minor cleanups in slot related code.

Fix a bunch of typos, and remove two superflous includes.

Author: Gurjeet Singh
Discussion: CABwTF4Wh_dBCzTU=49pFXR6coR4NW1ynb+vBqT+Po=7fuq5iCw@mail.gmail.com
Backpatch: 9.4

src/backend/replication/logical/logical.c

@@ -28,9 +28,6 @@
 
 #include "postgres.h"
 
-#include <unistd.h>
-#include <sys/stat.h>
-
 #include "miscadmin.h"
 
 #include "access/xact.h"

src/backend/replication/slot.c

@@ -661,7 +661,7 @@ ReplicationSlotsComputeRequiredLSN(void)
 /*
  * Compute the oldest WAL LSN required by *logical* decoding slots..
  *
- * Returns InvalidXLogRecPtr if logical decoding is disabled or no logicals
+ * Returns InvalidXLogRecPtr if logical decoding is disabled or no logical
  * slots exist.
  *
  * NB: this returns a value >= ReplicationSlotsComputeRequiredLSN(), since it
@@ -876,7 +876,7 @@ StartupReplicationSlots(void)
 }
 
 /* ----
- * Manipulation of ondisk state of replication slots
+ * Manipulation of on-disk state of replication slots
  *
  * NB: none of the routines below should take any notice whether a slot is the
  * current one or not, that's all handled a layer above.