dm raid: remove unnecessary discard limits for raid0 and raid10

Commit 29efc390 ("md/md0: optimize raid0 discard handling") and
commit d30588b2 ("md/raid10: improve raid10 discard request")
remove MD raid0's and raid10's inability to properly handle large
discards. So eliminate associated constraints from dm-raid's support.

Depends-on: 29efc390 ("md/md0: optimize raid0 discard handling")
Depends-on: d30588b2

 ("md/raid10: improve raid10 discard request")
Reported-by: Matthew Ruffell <matthew.ruffell@canonical.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>