Drop most deprecated code in cubicweb.web
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.