pidns: remove now unused find_pid function.

This one had the only users so far - the kill_proc, which is removed, so
drop this (invalid in namespaced world) call too.

And of course - erase all references on it from comments.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>