dm bufio: remove code that merges slab caches

All slab allocators can merge duplicate caches.  So dm-bufio doesn't
need extra slab merging logic.  Instead it can just allocate one slab
cache per client and let the allocator merge them.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>