s390/cpum_cf: simplify hw_perf_event_destroy()

To remove an event from the CPU Measurement counter facility
use the lock/unlock scheme as done in event creation. Remove
the atomic_add_unless function to make the code easier.

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