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.

gui/multiple_resolutions/project.godot

@@ -49,4 +49,5 @@ theme/default_font_multichannel_signed_distance_field=true
 [rendering]
 
 renderer/rendering_method="gl_compatibility"
+renderer/rendering_method.mobile="gl_compatibility"
 environment/defaults/default_clear_color=Color(0.133333, 0.133333, 0.2, 1)