Remove obsolete references to device-specific push rules

[jgarplind]

Checklist

• Tests written for new code (and old code if feasible) Should be covered by existing tests, if any
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

Been reading up on push rules, and specifically how the only supported (as per the spec) scope is global, e.g. here: matrix-org/matrix-spec#637

I also found this old PR which did the most important parts, but then ran across some parts that unnecessarily catered for a no longer supported use case. Figured it could be a good opportunity to make a minor contribution to a much appreciated repo.

I also considered the possibility to adjust the function signatures to either not take scope, or to force it to be the literal "global", but I'm not sure if that is desirable, with regards to API stability, etc.

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This PR currently has none of the required changelog labels.

A reviewer can add one of: T-Deprecation, T-Enhancement, T-Defect, T-Task to indicate what type of change this is, or add Type: [enhancement/defect/task] to the description and I'll add them for you.

[github-actions]

Thanks for opening this pull request, unfortunately we do not accept contributions from the main branch of your fork, please re-open once you switch to an alternative branch for everyone's sanity. See https://github.com/matrix-org/matrix-js-sdk/blob/develop/CONTRIBUTING.md