Remove validates_timeliness

This library has been abandoned, has been producing deprecation warnings and is only used to validate a single field so remove it and perform the validation ourselves.

This is of arguable value but the deprecation warnings were annoying me. Also, the validator is only tested by the model specs.

Any opinions?
Trying to make the intent of this more explicit ends up with the following:
```ruby
      if value.nil?
        original = record.attributes_before_type_cast[attribute.to_s]
        begin
          Date.parse(original) unless original.nil?
        rescue ArgumentError
          record.errors.add(attribute, 'invalid date')
        end 
        return
      end 
```
Hard to say if it's any better or not... Maybe just sticking in a comment would be the thing to do