Remove unused files for kryo

This commit removes `memmove_backward.S` and `memmove_base.S`.  These
are stale files that were not removed in the previous fix:

    commit c1938294
    Date:   Fri Aug 17 14:24:35 2018 +0800

        Fix memmove() with memcpy() optimization

Test: It builds.
Change-Id: I9c4dfe002c867a27c37b3b08e80cbbe71dd6489f