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

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>