perf session: Remove last reference to hists struct

Now perf_session doesn't require that the evsels in its evlist are hists
containing ones.

Tools that are hists based and want to do per evsel events_stats
updates, if at some point this turns into a necessity, should do it in
the tool specific code, keeping the session class hists agnostic.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jean Pihet <jean.pihet@linaro.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
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-cli1bgwpo82mdikuhy3djsuy@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>