This gets rid of all the code paths that we had marked as deprecated for

the Djblets 3.0 release. This is a fairly short list, consisting of some

functions that now require **kwargs, the old {% querystring_with %}

template tag, and WebAPIResource.generate_etag().
Along with the code, RemovedInDjblets30Warning has been removed, since

it's no longer going to be used.

Unit tests pass in both Djblets and Review Board.