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

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