Stop moving of a secondary to primary tag failing

This means that an existing "secondary" instance is updated to be "primary"
instead of attempting to create a new "primary" one, which would fail because of
the uniqueness constraint on the `SpecialistSector` model.
Remove duplicate dependency options.

The dependency set on the main `specialist_sectors` association will take care
of cleaning up the dependent models so is unnecessary on the other scoped
associations. This also saves against potential issues with AR attempting to
destroy non-existent instances (because they've already been cleaned up).
Whitespace.
Merge pull request #1763 from alphagov/replace-primary-specialist-sectors

Stop moving secondary to primary specialist sector from falling over