perf report: Use perf_evlist__force_leader to support '--group'

Since we created a new function perf_evlist__force_leader(), remove the
old code and use that new evlist method.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1526914666-31839-3-git-send-email-yao.jin@linux.intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>