perf cpumap: Remove initializations done in perf_cpu_map__alloc()

When extracting this patch from Ian's original patch I forgot to remove
the setting of ->nr and ->refcnt, no need to do those initializations
again as those are done in perf_cpu_map__alloc() already, duh.

Cc: Ian Rogers <irogers@google.com>
Fixes: 1f94479e ("libperf: Make perf_cpu_map__alloc() available as an internal function for tools/perf to use")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>