perf/core: Remove wrong barrier

The barrier and comment make no sense:

 - if what the barrier says is true, it should be wmb() but that
   should then be part of the arch driver, not the generic code.

 - if it is an SMP barrier, there must be a matching barrier, and
   there isn't one.

So kill it.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/events/core.c

@@ -2097,11 +2097,6 @@ event_sched_in(struct perf_event *event,
 		event->hw.interrupts = 0;
 	}
 
-	/*
-	 * The new state must be visible before we turn it on in the hardware:
-	 */
-	smp_wmb();
-
 	perf_pmu_disable(event->pmu);
 
 	perf_set_shadow_time(event, ctx, tstamp);