Remove all email signups code

This PR removes all the email signup module ab test code and associated files. The only thing remaining will now be [the `Id.emailSignup` JavaScript endpoint in `identity/api.js`](https://github.com/guardian/frontend/blob/master/common/app/assets/javascripts/modules/identity/api.js#L196).

This should fix the breaking build due to the test expiration.

![ugha6u0](https://f.cloud.github.com/assets/1170410/2169919/5df4a3e0-9563-11e3-9005-540c0595c7bb.gif)