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.

3d/particles/project.godot

@@ -25,5 +25,3 @@ window/stretch/aspect="expand"
 [rendering]
 
 textures/default_filters/anisotropic_filtering_level=4
-quality/screen_filters/msaa=3
-environment/default_environment="res://default_env.tres"