gdbserver: Reset current_thread when the thread is removed.

Reset current_thread and make sure 'remove_process' is used
after all associated threads have been removed first.

gdb/gdbserver/ChangeLog:
	* inferiors.c (thread_pid_matches_callback): New function.
	(find_thread_process): New function.
	(remove_thread): Reset current_thread.
	(remove_process): Assert threads have been removed first.