Remove support for encoded-call messaging from relay and runtime integration code

[svyatonik]

I'm opening this PR early (it'll be merged not earlier than on monday evening) to give those who are building using our codebase a chance to prepare. So we'll concentrate on supporting XCM dispatch in our bridge in next month(s). What it means:

• we've decided to drop support for encoded-call messaging from relay and runtime integration code, because it'll be too hard now to support the stuff that we're not going to use in the near future;
• we'll remove pallet-bridge-dispatch and pallet-token-swap (which depends on the former pallet) from our codebase;
• core pallets of our bridge (pallet-bridge-grandpa and pallet-bridge-messages) are staying and will likely keep the same API (this may be changed in next XCM-related milestones though). It means that if you're using encoded-calls messaging, nothing changes for your runtime, except that you won't be able to use following runtime-related crates from our repo: bp-message-dispatch, pallet-bridge-dispatch and bridge-runtime-common which will be changed to support XCM delivery. We'll make a tag before merging this PR and a comment in top-level readme, referencing this tag;
• headers/messages/headers+messages relay will likely keep supporting both options - XCM messaging and encoded-call messaging. However all subcommands that we were using for tests (send-message, encode-message, estimate-fee) will be changed to support XCM messaging.

Pinging guys from Darwinia, as I know you're using some of our crates: @AurevoirXavier @fewensa @hackfisher

[hackfisher]

@svyatonik We are replacing the offchain relay part using subquery indexing, including optimization for mandatory blocks and Justification. It will be a fork/different implementation, so I think relayer client part will be OK for us.

For on-chain part, as I understand, the changes will be related to the format of the bridge message, from dispatch call to unified XCM. We'll keep an eye on this, especially bridge-dispatch and see how to upgrade the protocols.

Currently using production bridges, Darwinia <> Crab, and Crab <> Crab Parachain is on the way.

[svyatonik]

@acatangiu It should be ready for review. The result of this PR:

• we have dropped encoded-call messaging. There may be some leftovers though, but in general there's no "official" runtime+relay support for that from now on;
• on our test deployments we are now generating + delivering simple blobs;
• dispatch of blob = simple logging for now. Later on, when we'll plugin XCM, we'll be using XCM dispatcher instead