Remove old Participate page model

[alanmoo]

It's been replaced by Participate2 and that's now being mocked in fake data generation. Let's get rid of the old one entirely.

[youriwims]

Removing this model is blocked by removing its usage in /network-api/networkapi/wagtailpages/translation.py. We're working on establishing a process to fully remove translation bindings in order to remove the model's import in translation.py thereby making it okay to completely remove the model in /network-api/networkapi/wagtailpages/models.py. This is also an issue with PeoplePage (Reference #2904 , #2898).
cc: @Pomax @cadecairos @patjouk