Remove tax disc a/b test and links to the original service

This pull request removes the A/B test we've been using on the tax disc start page. It also removes links to the original Directgov branded tax disc service.

We've been using the A/B test to raise the number of people using the Vehicle Tax beta service in steps. This beta has now been serving the majority of users since August. We should now remove the A/B test and make the more prominent position of the beta a permanent change. 

The original tax disc service is missing some of the newer features of the beta such as Direct Debit payment. It's also been disabled by the DVLA since 30 September. We should remove links to it to avoid confusing users with an alternative option which isn't available.

This pull request should be deployed alongside a change to content publisher to switch the service's URL to the  beta rather than the original directgov URL.