Remove unused DateValidator
Remove unused DateValidator

This hasn't been used since the big commit [1] where the bulk of the
specialist publisher code was removed.

Note that this was not obvious, because the validation was being
triggered by the `date` option in lines like this:

    validates :tribunal_decision_decision_date, presence: true, date: true

There are no such lines left in the code now.

[1]: 3162f0e72fd53bfd1cf87f5661bb198ff785940e

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher