perf unwind: Remove unnecessary test

perf unwind: Remove unnecessary test

If dwarf_callchain_users is false, then unwind__prepare_access() will
not set unwind_libunwind_ops so the remaining test here is sufficient.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: john keeping <john@metanate.com>
Link: http://lkml.kernel.org/r/20190815100146.28842-3-john@metanate.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>