screenshot: remove redundant mkdirp call

There is no reason to create the screenshot dir separately, because the
final path starts with it. So defer creating directories to the last
possible moment with all the information necessary.

player/screenshot.c

@@ -305,9 +305,6 @@ static char *gen_fname(struct mp_cmd_ctx *cmd, const char *file_ext)
             void *t = fname;
             dir = mp_get_user_path(t, ctx->mpctx->global, dir);
             fname = mp_path_join(NULL, dir, fname);
-
-            mp_mkdirp(dir);
-
             talloc_free(t);
         }