This removes the redundancy between `RelaxDepsNone` and
`RelaxDepsSome []` by removing `RelaxDepsNone`.

This way we avoid the risk of subtle bugs that can occur if the same
semantic value can be expressed in a non-unique way.

A further step to normalise the type would be to turn `[RelaxedDep]`
into `Set RelaxedDep`, but there is no operation that would
significantly benefit from that yet.