perf stat: Do not clean event's private stats

perf stat: Do not clean event's private stats

Mel reported stddev reporting was broken due to following commit:

	ccd89f57ffd5 ("perf stat: Introduce read_counters function")

This commit merged interval and overall counters reading into single
read_counters function.

The old interval code cleaned the stddev data for some reason (it's
never displayed in interval mode) and the mentioned commit kept on
cleaning the stddev data in merged function, which resulted in the
stddev not being displayed.

Removing the wrong stddev data cleanup init_stats call.

Reported-and-Tested-by: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: stable@vger.kernel.org # v4.2+
Fixes: ccd89f57ffd5 ("perf stat: Introduce read_counters function")
Link: http://lkml.kernel.org/r/1453290995-18485-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>