[NativeProcessLinux] Remove event mutex and clean functions using it

Summary:
Since the former-TSC events are now processed synchronously, there is no need for to protect them
with a separate mutex - all the actions are now guarded by the big m_threads_mutex.

With the mutex gone, the following functions, no longer have any purpose and were removed:
NotifyThreadCreate: replaced by direct calls to ThreadWasCreated
NotifyThreadStop: replaced by direct calls to ThreadDidStop
NotifyThreadDeath: folded into StopTrackingThread
ResetForExec: inlined as it consisted of a single line of code
RequestThreadResume(AsNeeded): replaced by direct calls to ResumeThread
StopThreads: removed, as it was never called

Test Plan: tests continue to pass

Reviewers: ovyalov, chaoren

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9603

llvm-svn: 237101