Remove unnecessary pg_config.h includes

pg_config.h is already included via c.h from postgres.h, individual
files in the backend should not include it separately (especially
not for improved editor support as some comments alluded to).

src/backend/access/appendonly/appendonlywriter.c

@@ -38,7 +38,6 @@
 #include "cdb/cdbdispatchresult.h"
 #include "cdb/cdbtm.h"
 #include "cdb/cdbutil.h"
-#include "pg_config.h"
 
 /*
  * GUC variables

src/backend/postmaster/alertseverity.c

@@ -13,7 +13,6 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
-#include "pg_config.h"  /* Adding this helps eclipse see that USE_SNMP is set */
 
 #include <fcntl.h>
 #include <signal.h>

src/backend/postmaster/sendalert.c

@@ -13,7 +13,6 @@
  *-------------------------------------------------------------------------
  */
 #include "postgres.h"
-#include "pg_config.h"  /* Adding this helps eclipse see that USE_SNMP is set */
 
 #include <fcntl.h>
 #include <signal.h>