perf evsel: Rename perf_evsel__open_per_*() to evsel__open_per_*()

As those are not 'struct evsel' methods, 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>

tools/perf/tests/openat-syscall.c

@@ -34,7 +34,7 @@ int test__openat_syscall_event(struct test *test __maybe_unused, int subtest __m
 		goto out_thread_map_delete;
 	}
 
-	if (perf_evsel__open_per_thread(evsel, threads) < 0) {
+	if (evsel__open_per_thread(evsel, threads) < 0) {
 		pr_debug("failed to open counter: %s, "
 			 "tweak /proc/sys/kernel/perf_event_paranoid?\n",
 			 str_error_r(errno, sbuf, sizeof(sbuf)));