Remove unused PoA<>Substrate bridge (#1210)

* Decouple the PoA bridge code from Rialto
* Remove Rialto PoA bridge code
* Remove relays/bin-ethereum code
* Remove relays/client-ethereum code
* Remove modules/ethereum code
* Remove modules/ethereum-contract-builtin code
* Remove PoA bridge documentation
* Remove primitives/ethereum-poa code
* Decouple Rialto from currency-exchange
* Fix building with runtime-benchmarks
* Fix should_encode_bridge_send_message_call test
    Because we removed some runtime modules/pallets, the
    substrate2substrate bridge pallet has a different index within
    the runtime so its calls have a different encoding.
    Update the test to use the new encoding.
* Update readme - no more PoA bridge
* Remove deployments/bridges/poa-rialto
    Also removes:
    - deployments/networks/eth-poa.yml
    - deployments/networks/OpenEthereum.Dockerfile
* Remove deployments/dev/poa-config
* Update deployments readme - no more PoA bridge
* Remove eth-related scripts
    Deletes:
    - deployments/networks/eth-poa.yml
    - scripts/run-openethereum-node.sh
* Remove poa-relay from gitlab-ci
* Dockerfiles to use substrate-relay as default
* Remove modules/currency-exchange code
* Remove primitives/currency-exchange code

Signed-off-by: acatangiu <[email protected]>