perf tools: Ditch the symbol_conf.nr_events global

perf tools: Ditch the symbol_conf.nr_events global

Since over time the places where we need to pass this got reduced
because we can obtain it from evsel->evlist->nr_entries, no need to have
this global anymore.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-ovhikrfj8pzdv93yq3gt6sei@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>