perf metricgroup: Add missing list_del_init() when flushing egroups list

So that at the end each of the entries have its list node struct cleared
and the egroup list head ends emptied.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-dxzj1ah350fy9ec0xbhb15b6@git.kernel.org


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