Now that grub2 supports btrfs, it seems best to drop this check.
Although I don't know about other boot loaders, and it's also possible
in some situations for d-i to choose to use grub-legacy, which would
fail on btrfs.
This is untested and unlikely to work without parted support.
Committing what I have since I don't trust btrfs to preserve the data. ;-)
Note to translators: Probably not worth wasting your time with this yet.
Have not run debconf-updatepo.
r62090
