Remove useless lstat() call in pg_rewind.

This is duplicative of an lstat that was just done by the calling
function (traverse_datadir), besides which we weren't really doing
anything with the results.  There's not much point in checking to
see if someone removed the file since the previous lstat, since the
FILE_ACTION_REMOVE code would have to deal with missing-file cases
anyway.  Moreover, the "exists = false" assignment was a dead store;
nothing was done with that value later.

A syscall saved is a syscall earned, so back-patch to 9.5
where this code was introduced.

Discussion: https://postgr.es/m/1221796.1599329320@sss.pgh.pa.us

Author: tglsfdc

src/bin/pg_rewind/filemap.c

@@ -349,8 +349,6 @@ process_target_file(const char *path, file_type_t type, size_t oldsize,
 					const char *link_target)
 {
 	bool		exists;
-	char		localpath[MAXPGPATH];
-	struct stat statbuf;
 	file_entry_t key;
 	file_entry_t *key_ptr;
 	filemap_t  *map = filemap;
@@ -362,16 +360,6 @@ process_target_file(const char *path, file_type_t type, size_t oldsize,
 	 * the source data folder when processing the source files.
 	 */
 
-	snprintf(localpath, sizeof(localpath), "%s/%s", datadir_target, path);
-	if (lstat(localpath, &statbuf) < 0)
-	{
-		if (errno != ENOENT)
-			pg_fatal("could not stat file \"%s\": %m",
-					 localpath);
-
-		exists = false;
-	}
-
 	if (map->array == NULL)
 	{
 		/* on first call, initialize lookup array */