perf evsel: Ditch perf_evsel__cmp(), not used for quite a while

In 4c358d5c ("perf stat: Replace transaction event possition check
with id check") all its uses were removed, so ditch it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>