perf evlist: Remove stale mmap read for backward

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 <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Acked-by: Wang Nan <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jin Yao <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>