Remove experimental dmraid support

Remove support for installing on dmraid devices. The dmraid code is upstream dead for more than 10 years, and semi-modern hardware does not need dmraid - instead, mdraid can support these devices.

d-i support for dmraid was always behind a boot flag. Seems like a low-risk change to me. Untested due to me lacking a build environment.

Also see https://bugs.debian.org/864423