Simplify ScroogeGen.

It used to support multiple compilers, but no longer needs to.
This change implements a TODO to rip out that support.

This will make a future change (to switch tool registration to use
the options system) easier.

Testing Done:
Ran the scrooge unittest.

Reviewed at https://rbcommons.com/s/twitter/r/1494/
Sync of https://github.com/pantsbuild/pants/commit/47ee133566c5a6eb2e34c7bc64a3801ee72a5a0f