invert worked at different address logic to match copy

Couldn't see why the overridden method `worked_at_different_address=` exists. I've tested various different cases without it and everything seems to work. I also noticed it's not included as a boolean as `no_acas_number` is but again everything seems to work but maybe I'm missing something.
Originally this was a computed property based on whether the respondent model had both addresses. ActiveRecord does type coercion and as this wasn't a column on the respondents table, the string value of the form input wasn't being coerced to a boolean. This would cause the form builder to not reuse the value when the page was redisplayed in the case of a validation error. 

Now that it's backed by a database column I think it's ok to remove.