Remove remote repository-access-through-pyro support
Modern methods such as the rqlcontroller cube + the cwclientlib
library are the way forward.
Closes #2919309.
What's new in CubicWeb 3.21?
============================
Code movement
-------------
The cubicweb.web.views.timeline module (providing the timeline-json, timeline
and static-timeline views) has moved to a standalone cube_
.. _cube: https://www.cubicweb.org/project/cubicweb-timeline
Deprecated code drops
---------------------
* the user_callback api has been removed; people should use plain
ajax functions instead
* the Pyro remote repository access method has been entirely removed
(emerging alternatives such as rqlcontroller and cwclientlib should
be used instead)