perf evsel: Remove superfluous nthreads system_wide setup in alloc_fd()

perf evsel: Remove superfluous nthreads system_wide setup in alloc_fd()

It's already setup in the only caller of this method in
perf_evsel__open(), right before calling perf_evsel__alloc_fd(), no need
to do it again.

Also it's better to have it out of the function before we move it to
libperf.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-1k8lhyjxfk7o8v4g3r7eyjc9@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>