libreboot-utils: simplify random tmpdir namegen

generalise it in rand.c because this logic will
be useful for other programs in the future.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/file.c

@@ -537,20 +537,6 @@ try_err(int loop_err, int errval)
 	return -1;
 }
 
-void
-free_and_set_null(char **buf)
-{
-	if (buf == NULL)
-		err_exit(EFAULT,
-		    "null ptr (to ptr for freeing) in free_and_set_null");
-
-	if (*buf == NULL)
-		return;
-
-	free(*buf);
-	*buf = NULL;
-}
-
 void
 open_on_eintr(const char *path,
     int *fd, int flags, mode_t mode,