[lldb] Remove support for SBHostOS threading functionality

As stated on Discourse*, these methods have been deprecated. I am
removing their implementation. They will now do nothing and return a
value indicating failure (where appropriate).
Due to the LLDB project's commitment to ABI stability at the SB API
layer, we cannot remove these symbols completely.

Discourse link: https://discourse.llvm.org/t/do-you-use-the-threading-functionality-in-sbhostos/71973