perf evlist: Remove stale mmap read for backward

perf_evlist__mmap_read_catchup() and perf_evlist__mmap_read_backward()
are only for overwrite mode.

But they read the evlist->mmap buffer which is for non-overwrite mode.

It did not bring any serious problem yet, because there is no one use
it.

Remove the unused interfaces.

Signed-off-by: Kan Liang <kan.liang@intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Wang Nan <wangnan0@huawei.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1516310792-208685-2-git-send-email-kan.liang@intel.com


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