perf: Remove unused struct perf_event::event_callback

This field might result from an older manual rebasing mistake.
We don't use it.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: "K. Prasad" <prasad@linux.vnet.ibm.com>

include/linux/perf_event.h

@@ -670,8 +670,6 @@ struct perf_event {
 
 	perf_callback_t			callback;
 
-	perf_callback_t			event_callback;
-
 #endif /* CONFIG_PERF_EVENTS */
 };