[lldb] Remove non-stop mode code

We added some support for this mode back in 2015, but the feature was
never productionized. It is completely untested, and there are known
major structural lldb issues that need to be resolved before this
feature can really be supported.

It also complicates making further changes to stop reply packet
handling, which is what I am about to do.

Differential Revision: https://reviews.llvm.org/D110553