account_steal_time: kill the unneeded account_group_system_time()

Impact: remove unnecessary accounting call

I don't actually understand account_steal_time() and I failed to find the
commit which added account_group_system_time(), but this looks bogus.
In any case rq->idle must be single-threaded, so it can't have ->totals.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>