[NativeProcessLinux] Remove the stop callback

Summary:
The stop callback is a remnant of the ThreadStateCoordinator. We don't need it now that TSC is
gone, as we know exactly which function to call when threads stop. This also removes some
stop-related functions, which were just forwarding calls to one another.

Test Plan: ninja check-lldb continues to pass

Reviewers: chaoren, ovyalov

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9531

llvm-svn: 236814