[runtimes] Remove all traces of the legacy testing configuration system

Now that all jobs have moved over to the new style of Lit configuration,
we can remove all traces of the legacy testing configuration system.
This includes:
- Cache settings that are not honored or useful anymore
- Several CMake options that were only useful in the context of the
  legacy Lit configuration system
- A bunch of Python support code that is not used anymore
- The legacy lit.cfg.in files themselves

Differential Revision: https://reviews.llvm.org/D134650