This reverts commit fab495a9e1442d99e922367f65f41de5eaa488eb.
As noted in
https://github.com/matrix-org/synapse/pull/13710#issuecomment-1298396007:
> We want to see this change land for the protocol's sake (and plan to
un-revert it) but want to give this a little more time before releasing
this.