staging/android/ion: delete dma_buf->kmap/unmap implemenation

There's no callers in-tree anymore.

For merging probably best to stuff this into drm-misc, since that's
where the dma-buf heaps will land too. And the resulting conflict
hopefully ensures that dma-buf heaps wont have a new ->kmap/unmap
implemenation.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Laura Abbott <labbott@redhat.com>
Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Laura Abbott <labbott@redhat.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: devel@driverdev.osuosl.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-5-daniel.vetter@ffwll.ch