Since 19556439 ("perf_counter: kerneltop: simplify data_head read")
we do not use it, and this was way back in 2009, remove it before some
other arch maintainer adds its implementation, like so many did,
needlessly :-)

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: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-3l2su9c58eaq4twjzrf9uu08@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>