Node Side: Remove `validation_code` runtime API

https://github.com/paritytech/polkadot/pull/3655 switches from `validation_code`, which queries the current version of code, to `validation_code_by_hash`. This, to my knowledge, is the last usage of that API and thus we can sunset it.

This issue is only about removing the node side code. We cannot remove the runtime API right away so that the older nodes will still be able to function.