Remove redundant code
Explicitly enable public action request formats

Instead of throwing 500 errors for requests for formats that are not
supported, return a 404. Actions that need to respond to formats other
than html must now explicitly enable them using the
enable_request_format helper.
Merge pull request #1090 from alphagov/61203922-handle-invalid-json-requests

Handle invalid json requests

https://www.pivotaltracker.com/story/show/61203922