dm vdo: Remove unused uds_compute_index_size

uds_compute_index_size() has been unused since it was added in
commit b46d79bd ("dm vdo: add deduplication index storage interface")

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>