Detach bindgen for next release

[CGMossa]

For various reasons, we have to make a new release of libR-sys.

This PR removes all uses of bindgen, and removes all features associated with generating bindings.
The currently available bindings in /bindings are those that we will have until #189 is addressed.

After this PR, the MSRV for libR-sys is at: 1.58.1.

[JosiahParry]

Thank you! I think we need move the deleted build contents into ./create-bindings.rs and in the Cargo.toml write exclude = ["create-bindings.rs"]

[CGMossa]

Thank you! I think we need move the deleted build contents into ./create-bindings.rs and in the Cargo.toml write exclude = ["create-bindings.rs"]

I would vote for keeping this PR clean. It removes code, and we publish it, without any clutter. Then after that, we'll make a release, and then we can bring those bits back in.

[JosiahParry]

I get that. Okie. Let's merge this so we can unblock CRAN fixes.

[JosiahParry]

Approved. Let's merge and work on the xtask