Stop using twisted.internet.defer.returnValue

[cjwatson]

defer.returnValue was only needed in Python 2; in Python 3, a simple return is fine.

twisted.internet.defer.returnValue is deprecated as of Twisted 24.7.0.

Most uses of returnValue in synapse were removed a while back; this cleans up some remaining bits.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[erikjohnston]

Thanks! Could you quickly fix the linting problems? They're just removing unused imports, but alas I don't have the ability to edit your PR directly.

[cjwatson]

Oh oops, I had the fix for those in my local copy but forgot to commit and push it. Fixed now, thanks.

[cjwatson]

I don't know what's going on with the sytest failures. I could be wrong but they look pretty unrelated to my change?

[erikjohnston]

I don't know what's going on with the sytest failures. I could be wrong but they look pretty unrelated to my change?

Ooops, looks like we broke CI yesterday! Have fixed.