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: NLin 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: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/cpu/perf_event.c

@@ -1571,9 +1571,6 @@ const struct pmu *hw_perf_event_init(struct perf_event *event)
  * callchain support
  */
 
-static DEFINE_PER_CPU(struct perf_callchain_entry, perf_callchain_entry_nmi);
-
-
 static void
 backtrace_warning_symbol(void *data, char *msg, unsigned long symbol)
 {