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/builtin-top.c

@@ -728,7 +728,7 @@ static void perf_event__process_sample(struct perf_tool *tool,
 
 	if (!machine) {
 		pr_err("%u unprocessable samples recorded.\n",
-		       top->session->hists.stats.nr_unprocessable_samples++);
+		       top->session->stats.nr_unprocessable_samples++);
 		return;
 	}
 
@@ -878,7 +878,7 @@ static void perf_top__mmap_read_idx(struct perf_top *top, int idx)
 			hists__inc_nr_events(&evsel->hists, event->header.type);
 			machine__process_event(machine, event);
 		} else
-			++session->hists.stats.nr_unknown_events;
+			++session->stats.nr_unknown_events;
 	}
 }