Remove usage of find_inferior in regcache_invalidate_pid

Remove usage of find_inferior in regcache_invalidate_pid

Replace with for_each_thread with pid filtering.
regcache_invalidate_one is not longer needed, as it was only used to
filter the pid.  We can call regcache_invalidate_thread directly.

gdb/gdbserver/ChangeLog:

	* regcache.c (regcache_invalidate_one): Remove.
	(regcache_invalidate_pid): use for_each_thread.