perf clang: Delete needless util-cxx.h header

perf clang: Delete needless util-cxx.h header

It was put in place just to make sure the 'new' C++ operator wouldn't
clash with some argument name in util.h, but there is not anymore any
such argument and also the reason stated for util.h to be included there
was to get the __maybe_unused definition, that is in linux/compiler.h,
so use that instead and nuke util-cxx.h.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: He Kuang <hekuang@huawei.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-1r5tvfnwiydjxhukgqs6bi11@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>