Remove the tax disc A/B test

The tax disc A/B test has been used to 
incrementally increase the number of visitors who
are presented with the Vehicle Tax beta as a
primary option.

The beta has been running at approx. 100% for the
last 3 months. We can now remove the A/B test and
make the change more permanent.

This commit removes the A/B test from the tax disc
start page and any associated assets.
Make the vehicle tax disc beta the primary option

As the A/B test was removed in 52d8fec, the Vehicle Tax beta will be
displayed as the secondary option as originally designed. 

This commit moves the beta back to the primary position it had during
the A/B test.
Remove links to the original tax disc service

The original tax disc service doesn't support the new Direct Debit
payment options and is missing some other more recent changes. The
majority of users have been using the beta service since August 2014
and this is where DVLA's development effort is now focussed.

As the original service was disabled by DVLA on 30 September, we should
remove links to it to avoid confusing users.
Merge pull request #690 from alphagov/remove-tax-disc-ab-test

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