block: Drop aio/cache consistency check from qmp_blockdev_add()

The TODO comment has been addressed a while ago and this is now checked
in raw-posix, so we don't have to special case this in blockdev-add any
more.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
