perf stat: Always keep perf metrics topdown events in a group

If any member in a group has a different cpu mask than the other
members, the current perf stat disables group. when the perf metrics
topdown events are part of the group, the below <not supported> error
will be triggered.

  $ perf stat -e "{slots,topdown-retiring,uncore_imc_free_running_0/dclk/}" -a sleep 1
  WARNING: grouped events cpus do not match, disabling group:
    anon group { slots, topdown-retiring, uncore_imc_free_running_0/dclk/ }

   Performance counter stats for 'system wide':

         141,465,174      slots
     <not supported>      topdown-retiring
       1,605,330,334      uncore_imc_free_running_0/dclk/

The perf metrics topdown events must always be grouped with a slots
event as leader.

Factor out evsel__remove_from_group() to only remove the regular events
from the group.

Remove evsel__must_be_in_group(), since no one use it anymore.

With the patch, the topdown events aren't broken from the group for the
splitting.

  $ perf stat -e "{slots,topdown-retiring,uncore_imc_free_running_0/dclk/}" -a sleep 1
  WARNING: grouped events cpus do not match, disabling group:
    anon group { slots, topdown-retiring, uncore_imc_free_running_0/dclk/ }

   Performance counter stats for 'system wide':

         346,110,588      slots
         124,608,256      topdown-retiring
       1,606,869,976      uncore_imc_free_running_0/dclk/

         1.003877592 seconds time elapsed

Fixes: a9a17902 ("perf stat: Ensure group is defined on top of the same cpu mask")
Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20220518143900.1493980-3-kan.liang@linux.intel.com


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