Remove Squeeze-only workaround for the HashType enum in Tails::IUK::TargetFile::Download
Type of work:
In the iuk repository,
lib/Tails/IUK/TargetFile/Download.pm reads: "FIXME: Squeeze's Moose::Util::TypeConstraints absolutely wants at least two members in an enum". We should remove the ugly placeholder in that enum.