Update/remove documentation on the 'revisiting backend'

[anjackson]

The documentation refers to a 'revisiting backend' (e.g. https://frontera.readthedocs.io/en/latest/topics/frontera-settings.html#revisiting-backend) but this appears to have been removed. I guess those parts of the documentation should be removed?

[sibiryakov]

Yes, these are outdated @anjackson.

[Prometheus3375]

Have the documentation updated? Quick search through sources shows that setting SQLALCHEMYBACKEND_REVISIT_INTERVAL is unused. I think this setting should be removed from default in sources and from documentation.