Remove references to physical tax disc

 As of October 1st, paper tax discs will be abolished

 - Remove reference to tax disc in /tax-disc, /make-a-sorn and /check-vehicle-tax
 - Move duplicate requirements to shared partial
Merge pull request #672 from alphagov/goodbye-paper-tax-discs

Goodbye paper tax discs (DO NOT MERGE UNTIL OCT 1ST)