s390/vtime: Get rid of redundant WARN_ON

in the cpu time accounting function vtime_account_irq_enter
(vtime_account_system) we use a WARN_ON_ONCE(!irqs_disabled()).
This is redundant as the function virt_timer_forward is always
called and has a BUG_ON(!irqs_disabled()).

This saves several nanoseconds in my specific testcase (KVM entry/exit)
and probably all other callers like (soft)irq entry/exit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>