Remove groups (#2234)
This API is due for removal in Synapse and has been deprecated for a very long time. People should move away from it soon, but just in case we'll declare this as a breaking change.
There is no impact on sync storage here: we happen to store the data in a way that is backwards-compatible for group-supporting clients, and the code guards against missing data from the stores. So, if someone were to revert, they'd be "safe" (probably lose all their group info, but the app wouldn't crash).
