Remove outdated include

In the wake of bfb9dfd93, there are no longer any stat() calls in
guc-file.l, but the work leading to dac048f71 did not get the memo.

Noted by Michael Paquier

Discussion: https://www.postgresql.org/message-id/Y2OosGi1Xh9x/lEn%40paquier.xyz

src/backend/utils/misc/guc-file.l

@@ -10,7 +10,6 @@
 #include "postgres.h"
 
 #include <ctype.h>
-#include <sys/stat.h>
 #include <unistd.h>
 
 #include "common/file_utils.h"