[lldb] [llgs] Remove not-really-used m_inferior_prev_state

Remove m_inferior_prev_state that's not suitable for multiprocess
debugging and that does not seem to be really used at all.

The only use of the variable right now is to "prevent" sending the stop
reason after attach/launch.  However, this code is never actually run
since none of the process plugins actually use eStateLaunching or
eStateAttaching.  Through adding an assert, I've confirmed that it's
never hit in any of the LLDB tests or while attaching/launching debugged
process via lldb-server and via lldb CLI.

Differential Revision: https://reviews.llvm.org/D128878
Sponsored by: The FreeBSD Foundation