perf tools: Ditch unused strchrnul() reimplementation

Remnants from the git codebase.

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: http://lkml.kernel.org/n/tip-kwaez3uxo1w9f8v5r7etl0w6@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>