dm cache: Remove unused functions in bio-prison-v1

dm_cache_size() and dm_cache_dump() are unused since commit
b29d4986 ("dm cache: significant rework to leverage dm-bio-prison-v2")

Remove them.

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