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

MusicPlayer.gd

@@ -28,9 +28,6 @@ var volume_slider: HSlider = $MusicPlayerContainer/VolumeSlider
 @onready
 var music_time_label: Label = $MusicPlayerContainer/MusicTimeLabel
 
-@onready
-var path: String = '/Users/sebastian/ResilioSync/Sorterat_test/Metal Gear Solid 4 - Guns of the Patriots/2-16 Metal Gear Saga.mp3'
-
 var play_icon: Texture = preload("res://icons/play_icon_light.svg")
 var pause_icon: Texture = preload("res://icons/pause_icon_light.svg")