Remove allow_no_prev_events option (MSC2716 cleanup)

[MadLittleMods]

Remove allow_no_prev_events option. This option is no longer used since we backed out the MSC2716 changes in matrix-org/synapse#15748 and is even mentioned as a follow-up task in the PR description there.

The allow_no_prev_events option was first introduced in matrix-org/synapse#11243 to support MSC2716 back in the day.

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)

[MadLittleMods]

Thanks for the review @reivilibre 🦇