perf stat: Remove print_mixed_hw_group_error

perf stat: Remove print_mixed_hw_group_error

print_mixed_hw_group_error will print a warning when a group of events
uses different PMUs.

This isn't possible to happen as parse_events__sort_events_and_fix_groups()
will break groups when this happens, adding the warning at the start
of perf of:

  WARNING: events were regrouped to match PMUs

As the previous mixed group warning can never happen, remove the
associated code.

Committer testing:

Before/after:

  acme@five:~$ perf stat -e '{cpu_core/cycles/,cpu_atom/cycles/}' sleep 1
  WARNING: events were regrouped to match PMUs

   Performance counter stats for 'sleep 1':

             424,895      cpu_atom/cycles/u
       <not counted>      cpu_core/cycles/u         (0.00%)

         1.011862314 seconds time elapsed

         0.000000000 seconds user
         0.003166000 seconds sys

  acme@five:~$

Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Levi Yun <yeoreum.yun@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Weilin Wang <weilin.wang@intel.com>
Link: https://lore.kernel.org/r/20250402201549.4090305-3-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>