Remove audit app

[jonathonherbert]

What does this change?

Removes the audit app, introduced as a little experiment way back in https://github.com/guardian/typerighter/pull/44.

We've got a story for testing rules pending in the rule manager client (e.g. #389), so this code is no longer needed, and removing it will speed up CI and reduce our dependency surface area.

How to test

• The app should build and run as before
• There should be no 'audit' link in the header of the checker UI:

[simonbyford]

You absolutely love to see it

Tested on CODE (as my local environment is broken at the moment) and everything looks great, nice one!