Remove unused function prototype

Commit aafc05de1b removed StartSlotSyncWorker() but mistakenly left
the prototype in slotsync.h.  Fix by removing.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/3F577953-A29E-4722-98AD-2DA9EFF2CBB8@yesql.se

src/include/replication/slotsync.h

@@ -27,7 +27,6 @@ extern char *CheckAndGetDbnameFromConninfo(void);
 extern bool ValidateSlotSyncParams(int elevel);
 
 extern void ReplSlotSyncWorkerMain(char *startup_data, size_t startup_data_len) pg_attribute_noreturn();
-extern int	StartSlotSyncWorker(void);
 
 extern void ShutDownSlotSync(void);
 extern bool SlotSyncWorkerCanRestart(void);