dm verity: remove unused mempool

Since commit 003b5c57

 ("block: Convert drivers to immutable biovecs"),
vec_mempool in struct dm_verity is no longer used.  Remove it and
related definitions.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>