Remove v0 node-side parachains code

[rphmeier]

This removes the following crates:

• polkadot-network
• polkadot-collator
• polkadot-availability-store
• (most of) polkadot-validation

polkadot-validation still remains as a simple stub that provides an empty set of candidates for inclusion in the set-heads inherent. Although v0 parachains runtime code was removed in #1501, there are still live networks (Polkadot, Kusama, Westend) which have it deployed, and they will require the set-heads inherent in order to proceed until a runtime upgrade enacts the removal on those networks.

At that point, we will be able to remove the polkadot-validation and polkadot-service crates entirely, replacing with v1 code.

[coriolinus]

Looks fine at a fairly cursory glance. Agree that it looks like the validation stub should just provide an empty set of heads for every block.

[gavofyork]

is the burnin happening?

(needs rebasing)

[rphmeier]

@gavofyork Yeah, it has burned in on Westend and Kusama correctly.

[rphmeier]

bot merge

[ghost]

Waiting for commit status.