perf tools metric: Don't include duration_time in group

The Memory_BW metric generates groups including duration_time, which
maps to a software event.

For some reason this makes the group always not count.

Always put duration_time outside a group when generating metrics.  It's
always the same time, so no need to group it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/20190628220737.13259-3-andi@firstfloor.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>