Per https://github.com/bazelbuild/rules_cc this still isn't necessary. There's no build-time enforcement, so usage is inconsistent/incorrect. Rather than letting this linger, remove it pending Bazel tooling enforcing it.
This should clean up our top level directory and the build patterns.
No non-mechanical edits here. Just injecting `toolchain/` and
`TOOLCHAIN_` and then running formatting tools.