Remove unnecessary CSRF token code

In general GOV.UK frontend apps can't use Cross Site Request Forgery (CSRF)
tokens because of caching

Requests to www.gov.uk pass through our Content Delivery Network (CDN) and our
own caching layer (Varnish). This means that the tokens are likely to be sent
to multiple users and hence be exposed to an attacker.

Whilst there are ways around the caching issue, we don't need CSRF protection
because we aren't providing any way for requests to change resource/server
state, so we don't need to worry about attackers impersonating users.

http://guides.rubyonrails.org/security.html#cross-site-request-forgery-csrf