s390/cpum_sf: Remove unused defines REG_NONE and REG_OVERFLOW

Member hw_perf_event::reg.reg is set but never used, so remove it.
Defines REG_NONE and REG_OVERFLOW are not referenced anymore.
The initialization to zero takes place in function
perf_event_alloc() where
  ...
  event = kmem_cache_alloc_node(perf_event_cache,
		                GFP_KERNEL | __GFP_ZERO, node);
  ...

makes sure memory allocated for the event is zero'ed.
This is done in the kernel's common code in kernel/events/core.c
The struct perf_event contains member hw_perf_event as in

  struct perf_event {
   ....
   struct hw_perf_event hw;
   ....
  };

This contained sub-structure is also initialized to zero.
No functional change.

Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>