Refactor `RRSC`

[0xbillw]

Motivation:

1. The code related to RRSC consensus is mostly a direct reference to BABE consensus with, the only difference being the implementation of its own NposSolver.
2. Based on the principle of prioritizing extensibility, modifying NposSolver alone can meet the requirements of RRSC.
3. Implementing a consensus mechanism by copying code introduces additional bugs and work for maintenance and upgrades.

Risks:

1. The current CESS testnet is long-term maintained, and it is necessary to consider whether this update will risk bricking the chain.

Steps:

1. Extract the existing implementation code related to NposSolver into a separate crate.
2. Modify the parts of the runtime and client that use RRSC to use BABE instead.
3. Delete the original RRSC code.