dm cache: revert "prevent corruption caused by discard_block_size > cache_block_size"

This reverts commit d132cc6d9e92424bb9d4fd35f5bd0e55d583f4be because we
actually do want to allow the discard blocksize to be larger than the
cache blocksize.  Further dm-cache discard changes will make this
possible.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>