Revert "Update tax-disc start to reduce load on beta service"

This reverts alphagov/frontend#675, which added a hot-fix to reduce traffic to the tax-disc service by showing an "unavailable" message to 20% of users.

The tax-disc service is handling traffic much better now and DLVA are happy to return to 100% traffic to the service, so this completely undoes the previous change.

cc @daibach