This includes both Python and JavaScript code. Most code has been
deprecated since a very long time (versions 3.1x mostly, more than 5
years ago, and a few things until 3.24).

There are still a few deprecated things, most of which are actually used
within cubicweb (or in cubes we depend on for tests). I don't know what
to do with these...

Removed a couple of unused imports along the way.

Changelog entry will come in a later changeset.