perf session: There is no need for a per session hists instance

It was being used just for its stats member, so ditch session->hists and
use just what is needed, session->stats.

This completes the move support multiple events in the hists layer, the
last user of session->hists was 'perf diff' but Jiri Olsa has fixed that
some time ago.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-pimk92kek8kcp4dmb1jakoro@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/session.h

@@ -34,11 +34,7 @@ struct perf_session {
 	struct rb_root		machines;
 	struct perf_evlist	*evlist;
 	struct pevent		*pevent;
-	/*
-	 * FIXME: Need to split this up further, we need global
-	 *	  stats + per event stats.
-	 */
-	struct hists		hists;
+	struct events_stats	stats;
 	int			fd;
 	bool			fd_pipe;
 	bool			repipe;