The main use case was to support custom versions of Godot through a convoluted build process.
With the new feature 'custom-godot', the user can directly generate bindings from a Godot version of choice.
However, godot-rust is not designed to be used without the GDNative API, so there is not much point in keeping that feature.

More in line with the package names. User-facing existing ones on 'gdnative' are not affected.

Reason: --features on a workspace level is not supported before.
Further complicating CI scripts, while we already have Rust 1.57, is questionable.

Also removes the custom-built clamp(), which was introduced as f32::clamp() in Rust 1.50.