Remove support for unstable MSC3882

[hughns]

The following client PRs should be merged ahead of this:

• Support for stable MSC3882 get_login_token element-hq/element-android#8299
• Use stable get_login_token and remove unstable MSC3882 support matrix-react-sdk#11001
• Support for stable MSC3882 get_login_token matrix-js-sdk#3416
• Support for stable MSC3882 get_login_token matrix-ios-sdk#1791

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.
• Pull request includes a sign off
• Code style is correct
  (run the linters)

[clokep]

For reference, #15388 landed in what will become Synapse v1.86.0. I think we'll want to keep the unstable version until at least Synapse v1.88.0.