Remove mustache JavaScript

It was previously being used for rendering ajax responses. That
functionality has since been removed [frontend/2c5ee617][1] but the
library has been left around.

[1]: https://github.com/alphagov/frontend/commit/2c5ee6172ab8c42508bcc98d8d15d300070a8539
Merge pull request #397 from alphagov/remove-mustache

Remove mustache JavaScript