Remove unused function declaration

It appears that check_track_commit_timestamp was declared but has never
been defined in our code base.  Likely this is just leftover cruft from
a development version of the original patch to add commit timestamps.

Let's just remove the useless declaration.  The inclusion of guc.h also
seems surplus to requirements.

Author: Andrey Lepikhov
Discussion: https://postgr.es/m/f49aefb5-edbb-633a-af07-3e777023a94d@postgrespro.ru

src/include/access/commit_ts.h

@@ -15,14 +15,10 @@
 #include "datatype/timestamp.h"
 #include "replication/origin.h"
 #include "storage/sync.h"
-#include "utils/guc.h"
 
 
 extern PGDLLIMPORT bool track_commit_timestamp;
 
-extern bool check_track_commit_timestamp(bool *newval, void **extra,
-										 GucSource source);
-
 extern void TransactionTreeSetCommitTsData(TransactionId xid, int nsubxids,
 										   TransactionId *subxids, TimestampTz timestamp,
 										   RepOriginId nodeid);