Remove usage of find_inferior in find_lwp_pid

Remove usage of find_inferior in find_lwp_pid

Replace with find_thread.  We could almost use find_thread_ptid, except
that find_lwp_pid uses the pid of the input ptid of the lwp is 0, so the
behavior is not quite the same.

gdb/gdbserver/ChangeLog:

	* linux-low.c (same_lwp): Remove.
	(find_lwp_pid): Use find_thread.