xen/gntdev-dmabuf: Ditch dummy map functions

There's no in-kernel users for the k(un)map stuff. And the mmap one is
actively harmful - return 0 and then _not_ actually mmaping can't end
well.

Acked-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org
Link: https://patchwork.freedesktop.org/patch/msgid/20191118103536.17675-14-daniel.vetter@ffwll.ch