Remove parachains mentions from Substrate repository

[tomusdrw]

Re: paritytech/substrate#9833 (comment)

The beefy-mmr pallet contains now functions and fields specific to parachains, hence it does not necessarily fit Substrate repository.

IMHO the best way forward with this would be to generalize parachains to something like "leaf extra data". This extra data would have it's own Provider passed in Config. The data returned by Provider can have two flavors:

1. A single blob - include as is.
2. A collection - merkelize and include hash in the leaf.

[seunlanlege]

This makes sense, we could just call it extras and have it be a simple blob of bytes with a bounded len?

[tomusdrw]

I think beefy-mmr should probably be moved to polkadot repository since it contains payload that is specific to polkadot. I'd also suggest to extract primitives that are beefy-mmr/polkadot specific, like concrete MmrLeaf.

[acatangiu]

Fixed by paritytech/substrate#10664 and paritytech/polkadot#4719.