perf evsel: Rename __perf_evsel__sample_size() to __evsel__sample_size()

As it is a 'struct evsel' related method, not part of tools/lib/perf/,
aka libperf, to whom the perf_ prefix belongs.

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>