Remove a lock acquisition from ProcessWindows::OnExitProcess.

This was causing a race condition where DoDestroy() would acquire
the lock and then initiate a shutdown and then wait for it to
complete.  But part of the shutdown involved acquiring the same
lock from a different thread.  So the main thread would timeout
waiting for the shutdown to complete and return too soon.

The end result of this is that SBProcess::Kill() was broken on
Windows.

llvm-svn: 225297