[lldb] Eliminate InitializePythonRAII::InitializeThreadsPrivate (NFC) (#151780)
The behavior of thread initialization changed in Python 3.7. The minimum
supported Python version is now 3.8. That means that
`PyEval_ThreadsInitialized` always returns true and `PyEval_InitThreads`
is never called.
The helper function existed to coordinate initializing the threads and
acquiring the GIL, which is no longer necessary. With that, there's no
point in having the helper at all. This PR eliminates the function and
inlines to GIL acquisition into the caller.