playlist: don't add the base path twice

Loading a playlist with --playlist from a sub-directory added the
playlist's base path twice: one time in the playlist demuxer, and then
again in playlist_parse_file(). The latter function is used only for
--playlist, so it worked when loading the playlist directly.

(This is probably a mess-up when the MPlayer playlist parsers were
replaced with newer code.)

CC: @mpv-player/stable

common/playlist.c

@@ -286,9 +286,6 @@ struct playlist *playlist_parse_file(const char *file, struct mpv_global *global
     if (ret && !ret->first)
         mp_warn(log, "Warning: empty playlist\n");
 
-    if (ret)
-        playlist_add_base_path(ret, mp_dirname(file));
-
     talloc_free(log);
     return ret;
 }