Remove our HTML format in django-rest-framework

[peterbe]

We learned in this PR that when we upgraded some things the only way we noticed that viewing the django-rest-framework rendering in HTML was by running it locally and opening it in a browser. Sadness.

The testing of the rendering of HTML via django-rest-framework should be part of the test suite.

[peterbe]

Probably safest to do this after https://github.com/mozilla/normandy/issues/1205

[jaredlockhart]

The HTML support of DRF is a nice to have and should not be tested.