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