Don't include rendering_app with individual routes.

This only needs to be set on the item, not on the individual routes (see
https://github.com/alphagov/content-store/blob/master/doc/input_examples/generic.json).
This value was being ignored, but it's best to remove it to avoid any
confusion.
Merge pull request #145 from alphagov/fix_route_registration

Don't include rendering_app with individual routes.