perf tools: Remove duplicate struct forward declarations

'struct evlist' has been declared at 10th line.

'struct comm' has been declared at 15th line.

Remove the duplicates
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: kael_w@yeah.net
Link: http://lore.kernel.org/lkml/20210325043947.846093-1-wanjiabing@vivo.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/metricgroup.h

@@ -9,7 +9,6 @@
 
 struct evlist;
 struct evsel;
-struct evlist;
 struct option;
 struct rblist;
 struct pmu_events_map;

tools/perf/util/thread-stack.h

@@ -16,7 +16,6 @@ struct comm;
 struct ip_callchain;
 struct symbol;
 struct dso;
-struct comm;
 struct perf_sample;
 struct addr_location;
 struct call_path;