Cleanup: bindgen library: do not depend on clang, do not suggest source package

Right now librust-bindgen-dev depends on clang and recommends rust-bindgen-cli.

Being rust source code and not invoking clang (the compiler) in its build.rs, librust-bindgen-dev does not actually depend on clang. Whatever clang dependencies it needs it should inherit from librust-clang-sys-dev (and it does indeed). librust-bindgen-dev's autopkgtests do not seem to be affected by removing this dependency (either because they just don't need it, or because, again, librust-clang-sys-dev already pulls them in).

AFAIK recommending a source package like rust-bindgen-cli doesn't do anything. It could make sense for librust-bindgen-dev to recommend the rust library package built from rust-bindgen-cli, but there is none as librust-bindgen-dev itself is the library package for the bindgen binary. So I'd just drop this Recommends. I see analogous Breaks and Replaces were already removed from rust-bindgen-cli in commit aaa8d10f.

Asking review to @werdahias since AFAICS he's the one that introduced these changes.