UPSTREAM: dm crypt: remove unused io_pool and _crypt_io_pool

(cherry pick from commit 94f5e024)

The previous commit ("dm crypt: don't allocate pages for a partial
request") stopped using the io_pool slab mempool and backing
_crypt_io_pool kmem cache.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Bug: 25392275
Change-Id: I115f909831f462c750cb3d3c04205d09b211fbf2