Remove non-Tendermint backends

[kostko]

We should remove the following non-Tendermint backends as they have become a maintenance burden and are only used in some tests:

• roothash,
• registry,
• epochtime,
• committee scheduler,
• beacon,
• staking.

Going forward we should still maintain clear separation between a consensus backend (e.g., Tendermint) and our internal backends.

Requires #1748.

[Yawning]

While we're at this, every Backend should be a BlockBackend where appropriate.

[kostko]

Yeah we should just get rid of BlockBackend and require height annotations in Backend.