Get rid of the kill_pgrp_info() function

There's only one caller left - the kill_pgrp one - so merge these two
functions and forget the kill_pgrp_info one.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Reviewed-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>