hw-breakpoints: Fix unused function in off-case

bp_perf_event_destroy() is unused in its off-case version, let's
remove it to fix the following warning reported by Stephen
Rothwell in linux-next:

  kernel/perf_event.c:4306: warning: 'bp_perf_event_destroy' defined but not used

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <1259180453-5813-1-git-send-email-fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -4303,10 +4303,6 @@ void perf_bp_event(struct perf_event *bp, void *data)
 		perf_swevent_add(bp, 1, 1, &sample, regs);
 }
 #else
-static void bp_perf_event_destroy(struct perf_event *event)
-{
-}
-
 static const struct pmu *bp_perf_event_init(struct perf_event *bp)
 {
 	return NULL;