Remove the prospective-parachains subsystem from collators (#4471)
Implements https://github.com/paritytech/polkadot-sdk/issues/4429 Collators only need to maintain the implicit view for the paraid they are collating on. In this case, bypass prospective-parachains entirely. It's still useful to use the GetMinimumRelayParents message from prospective-parachains for validators, because the data is already present there. This enables us to entirely remove the subsystem from collators, which consumed resources needlessly Aims to resolve https://github.com/paritytech/polkadot-sdk/issues/4167 TODO: - [x] fix unit tests