Remove unneeded includes of <sys/stat.h>

Since bfb9dfd, none of the files updated in this commit have any stat()
calls, so these inclusions are not necessary, for the same reasons as
233cf6e.

Per discussion with John Naylor.

Discussion: https://postgr.es/m/CAFBsxsGGGX7KD6RxbNoSJzuSc8Gz3hOxcfhTOMLB_hJcm68dKQ@mail.gmail.com

src/backend/access/heap/rewriteheap.c

@@ -102,7 +102,6 @@
  */
 #include "postgres.h"
 
-#include <sys/stat.h>
 #include <unistd.h>
 
 #include "access/heapam.h"

src/backend/storage/file/copydir.c

@@ -20,7 +20,6 @@
 
 #include <fcntl.h>
 #include <unistd.h>
-#include <sys/stat.h>
 
 #include "common/file_utils.h"
 #include "miscadmin.h"

src/timezone/pgtz.c

@@ -14,7 +14,6 @@
 
 #include <ctype.h>
 #include <fcntl.h>
-#include <sys/stat.h>
 #include <time.h>
 
 #include "common/file_utils.h"