Remove outdated backwards compat code

[m-vdb]

Fixes #6487

Proposed changes:

• The code changed in this PR was introduced for backwards compat reason, but has since evolved. In all the cases I think we can get rid of the various behaviours introduced (argumenting on each of them in the PR to make the review easier)

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)

[m-vdb]

I need to figure out why tests are currently failing

[wochinge]

@m-vdb Feel free to request review from me then 👍🏻 (instead of the entire squad)

[m-vdb]

@wochinge after some search I think I understand why the tests are currently failing, but I don't know how to fix this yet. Maybe you have an idea. The different event instances should have a metadata attribute, but for some reason, after they're deserialised via jsonpickle, this doesn't work (hence those attribute errors). I suspect some sort of cache in jsonpickle. But I'm not quite sure here 🤔 does this sound familiar?

These tests are failing, and at line 149 we deserialise events through a helper from conftest.

[wochinge]

Re the failing tests:

1. The old pickled dialogue files don't have the metadata property
2. Jsonpickle reloads the pickled dialogue without using the constructor (e.g. here, "When a class instance is unpickled, its init() method is usually not invoked.")
3. There is no attribute metadata as we have never assigned this attribute in the first place

As far as I can see from the events and tracker_store modules we are never restoring a tracker using pickle or jsonpickle which in turn means that the jsonpickling is no functionality which we need to test. You could

• re-dump the tracker states via json.dumps(dialogue.as_dict()) and then use json.loads to the load the tracker and execute the (actual) test.
• drop the persisted dialogues and inline them as list of events in the code. This way they are closer to the test code and we don't use code to serialize / deserialize trackers which we don't use anywhere else (I think I prefer this one)

[m-vdb]

@wochinge I've addressed your comment regarding test data in this commit: 12262cd. One thing that I wasn't sure about: where to put the test data. I've moved it in conftest.py as other tests depend on it (as you'll see in the commit).