refactor($compile): remove preAssignBindingsEnabled leftovers

Now that we don't need to support `preAssignBindingsEnabled` (removed in #15782),
complexity introduced in `$controller` by #7645 can be removed.

One difference with the previous implementation is that all non-ES2015-class controller instances
were available on the element before calling their constructors. Now it depends on the relative
order of controllers. Controller constructors shouldn't be used to access other controllers
(e.g. via `$element.controller(directiveName)`). The recommended way is to use the `require`
property of the directive definition object and the life cycle hooks `$onChanges` or `$onInit`.

See
https://docs.angularjs.org/api/ng/service/$compile#-require-
https://docs.angularjs.org/api/ng/service/$compile#life-cycle-hooks

Closes #16580