Disable aggregation bundling on `/sync` responses (#11583)

* Disable aggregation bundling on `/sync` responses

A partial revert of #11478. This turns out to have had a significant CPU impact
on initial-sync handling. For now, let's disable it, until we find a more
efficient way of achieving this.

* Fix tests.

Co-authored-by: Patrick Cloke <patrickc@matrix.org>