lang/rust: Clean up more things

• Drop pre-install target: make restage should be run after a failed make stage to wipe out and repopulate the staging directory. No need to hack around this.
• Drop RUST_MANIFESTS, it is only one value after r490852 and now only used once in post-install.
• Drop RUST_CHANNEL. It is only used in do-configure.
• Drop LLNEXTGEN option. There are no references to LLNextgen in the Rust sources, so it appears to not be used for anything anymore.