Removing tax-disc html.erb page and associated files

These files were duplicated to create the vehicle-tax page. Now vehicle-tax is live they are no longer needed.
Removing references to tax-disc from scss, controller and rendering test
Update tax-disc links on homepage
Update image link to point to vehicle-tax
Updating links and tests for check-vehicle-tax
Updating links and tests for make-a-sorn
Update background image links in css files
Merge pull request #700 from alphagov/remove-old-tax-disc-stuff

Removing tax-disc.html.erb and associated files