Revert "dm raid: fix discard limits for raid1 and raid10"

This reverts commit e0910c8e.

It causes problems :(

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Mike Snitzer <snitzer@redhat.com>
Cc: Zdenek Kabelac <zkabelac@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>