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