Remove to-be-deprecated rules iterable_contains_unrelated_type and list_remove_unrelated_type

[srawlins]

These two rules will be deprecated (in dart-lang/sdk#58958) in favor of collection_methods_unrelated_type, which I'd also like to add to package:lints (#787).

[devoncarew]

What SDK version would the rules be deprecated in (and the new rule be available)?

[srawlins]

The old rules will be deprecated in Dart 3.1 at the earliest. The new rule is available in Dart 2.19.0.

I'm not sure whether the right practice is to deprecate (which results in new diagnostics in analysis_options.yaml files) first or to remove from package:lints first.

[devoncarew]

Hmm, from a previous discussion in this repo, I believe that the deprecations wouldn't show as diagnostics for users? Since the user's analysis options file would reference this one. So we likely have a freer hand wrt timing here (esp. as this package already has a lower bounds of 3.0.0-417).

[srawlins]

The rules are now deprecated and should be removed from this repo.

[devoncarew]

Removals approved.