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

MainWindow.tscn

@@ -1,7 +1,6 @@
 [gd_scene format=3 uid="uid://xwq863o6uvsu"]
 
 [ext_resource type="Script" uid="uid://b8f4g15cas2j2" path="res://MainWindow.gd" id="1_eu0t5"]
-[ext_resource type="PackedScene" uid="uid://b16on0oscg1bv" path="res://CharacterSelect.tscn" id="2_76kf4"]
 [ext_resource type="Script" uid="uid://elgn7gkxgagq" path="res://InsperationScrollContainer.gd" id="2_gxtxm"]
 [ext_resource type="Texture2D" uid="uid://bcfmpd7h512ef" path="res://icons/person_add_light.svg" id="5_31tjv"]
 [ext_resource type="PackedScene" uid="uid://ds15cgsf8vpvc" path="res://MusicPlayer.tscn" id="5_emn36"]
@@ -167,15 +166,6 @@ label_settings = SubResource("LabelSettings_3m52w")
 horizontal_alignment = 1
 autowrap_mode = 2
 
-[node name="CharacterSelect" parent="." unique_id=2079506802 instance=ExtResource("2_76kf4")]
-visible = false
-top_level = true
-layout_mode = 0
-offset_left = 616.0
-offset_top = 264.0
-offset_right = 776.0
-offset_bottom = 424.0
-
 [node name="MusicPlayer" parent="." unique_id=1990459832 instance=ExtResource("5_emn36")]
 layout_mode = 0
 anchors_preset = 0