This is part 2 of 4, the main part. It removes the backend and the code in
`BaseFeature` and feature subclasses that hook into it. As a consequence it also
makes the other following changes:
* Remove `QuickSuggest.backend` (eventually I'd like to add a Merino backend and
then add a new `QuickSuggest.backends` list that has the Rust, Merino, and ML
backends)
* Add `QuickSuggest.config`
* Replace uses of `QuickSuggest.backend.config` and `QuickSuggest.jsBackend.config`
with `QuickSuggest.config`
* Remove the `quickSuggestRustEnabled` Nimbus variable
* Update most remaining tests so that they do not assume the JS backend exists
I left the `browser.urlbar.quicksuggest.rustEnabled` pref. I would like to
preserve the ability to toggle off Rust suggestions while keeping other parts of
Suggest enabled even if we don't need it now. That seems like a wise thing to do
and is similar to how both Merino and the ML backends can be toggled separately.
Depends on D231465
Differential Revision: https://phabricator.services.mozilla.com/D231466
2033165d91