s390/cpum_sf: Do not re-enable event after deletion

Event delete removes an event from the event list, but common
code invokes the PMU's enable function later on. This happens
in event_sched_out() and leads to the following call sequence:

  event_sched_out()
  +--> cpumsf_pmu_del()
  +--> cpumsf_pmu_enable()

In cpumsf_pmu_enable() return immediately when the event is not
active. Also remove an unneeded if clause. That if() statement
is only reached when flag PMU_F_IN_USE has been set in
cpumsf_pmu_add(). And this function also sets cpuhw->event
to a valid value.

Remove WARN_ON_ONCE() statement which never triggered.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>