rust: remove support for `re2` With the performance issues with `regex` figured out and fixed in previous patches and `regex` newly gaining support for empty alternations, there is no reason to keep `re2` around anymore. It's only *marginally* faster at creating the regex which saves at most a couple of ms, but gets beaten by `regex` in every other aspect. This removes the Rust/C/C++ bridge (hooray!), the `with-re2` feature, the conditional code that goes with it, the documentation and relevant part of the debug/module output. Differential Revision: https://phab.mercurial-scm.org/D8594