Syncs our lints with https://github.com/dart-lang/linter/blob/master/example/all.yaml:
* removed the deprecated `iterable_contains_unrelated_type` and `list_remove_unrelated_type` lints (their replacement `collection_methods_unrelated_type` was already enabled for us)
* enabled the new `no_self_assignments` and fixed one issue triggered by the lint.