dm-crypt: remove io_pool

Remove io_pool and _crypt_io_pool because they are unused.

CRs-fixed: 670391
Change-Id: I71400ecda66902c56c3af981e8b739f156db1e27
Signed-off-by: Mikulas Patocka <mpatock@redhat.com>
Patch-mainline: dm-devel @ 04/05/14, 14:08
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>