Remove unused code

- The old character select scene, which was never shown but loaded
  every character image at startup
- Unused SyncResponse and ProgressResponse classes
- Playlist.set_last_played_answered and select/local-loading code in
  the new character select, none of which were called
- Settings.delete_player_children, identical to delete_children
- An unused variable holding a local file path

playlist.gd

@@ -31,10 +31,6 @@ func get_preloaded_songs() -> int:
 func should_preload_more_songs() -> bool:
 	return number_of_tracks_to_preload > get_preloaded_songs()
 
-func set_last_played_answered() -> void:
-	if song_object_array[latest_played_song].has_played && !song_object_array[latest_played_song].is_answered:
-			song_object_array[latest_played_song].is_answered = true
-
 func song_is_answered(song_no: int = currently_playing_song) -> void:
 	# Nothing is playing yet (-1); a negative index would answer the last, unplayed song
 	if song_no < 0 || song_no >= song_object_array.size():