gdbserver: remove lwpid_of(thread)

This function doesn't seem so useful.  Use `thread_info::id::lwp`
directly.

Change-Id: Ib4a86eeeee6c1342bc1c092f083589ce28009be1
Reviewed-By: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
8 files changed