[bolt] Clean up lit site cfg files a bit

* Stop setting config.python_executable in Unit/lit.site.cfg.py.in.
  All other projects only set this in the main lit config, not in the
  one for unit tests. (Unit tests don't spawn Python.)
* Set config.python_executable to Python3_EXECUTABLE in main
  lit.site.cfg.py.in instead of PYTHON_EXECUTABLE. All other files
  did this in c4c3883b.
* Stop setting enable_abi_breaking_checks, enable_backtrace, enable_shared.
  Nothing in bolt's tests (or in lit) reads them.

Differential Revision: https://reviews.llvm.org/D138299