[lldb] [Process/FreeBSDRemote] Remove thread name caching

Remove the thread name caching code.  It does not handle the possibility
of thread name changing between requests, therefore breaking
TestGdbRemoteThreadName.  While technically we could cache the results
and reset the cache on resuming process, the gain from doing that
does not seem worth the effort.

Differential Revision: https://reviews.llvm.org/D90863