perf: Remove unused variable

This fixes the following build warning introduced by the
callchain rework:

  arch/x86/kernel/cpu/perf_event.c:1574: warning: ‘perf_callchain_entry_nmi’ defined but not used

Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
LKML-Reference: <1282718949.16443.75.camel@minggr.sh.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>