Dependency: Remove dead 'isSCM' code from Version
Dependency: Remove dead 'isSCM' code from Version

Before `Dependency` was made to use `std.sumtype`,
the code was piggybacking on a field `VersionRange m_range` to store
the git reference whenver the `Dependency` was a `Repository`.
Since this is no longer the case, we can remove this exception from `Version`.

Geod24/dub