Remove usage of find_inferior in lynx_mourn

Remove usage of find_inferior in lynx_mourn

Replace it with for_each_thread with pid filtering.  We can remove
lynx_delete_thread_callback and pass remove_thread directly.

I can't build/test this change, but it should be obvious enough.

gdb/gdbserver/ChangeLog:

	* lynx.low (lynx_delete_thread_callback): Remove.
	(lynx_mourn): Use for_each_thread.