Remove old and unused project settings, update various demos for 4.2 (#1024)

- Move all demo projects that don't require Forward+/Mobile-only features
  to the Compatibility rendering method. This improves performance significantly
  on low-end devices and ensures visuals are identical to a web export
  of the demo.
- Set deadzone on all inputs to 0.2 for better gamepad usability.
- Remove reliance on `default_env.tres` to use built-in Environment
  resources in the main scene instead (which follows the preview environment
  workflow).
- Remove notices pointing to GDNative or VisualScript, since both were
  removed in 4.0.
- Various bug fixes and usability tweaks to 10+ demos.

2d/physics_platformer/enemy/enemy.gd

@@ -1,4 +1,5 @@
-class_name Enemy extends RigidBody2D
+class_name Enemy
+extends RigidBody2D
 
 
 const WALK_SPEED = 50
@@ -13,8 +14,6 @@ var _state := State.WALKING
 var direction := -1
 var anim := ""
 
-var Bullet := preload("res://player/bullet.gd")
-
 @onready var rc_left := $RaycastLeft as RayCast2D
 @onready var rc_right := $RaycastRight as RayCast2D