Rococo AH: cleanup storage

[ggwpez]

Follow up on #4414 to clean up the old storage.

[bkchr]

I ignored this on purpose, because these chains are going to stop in the hopefully near future any way :D

[kianenigma]

How about a

pub struct DetectDanglingStorage<Exceptions: Get<Vec<u8>>>;
impl OnRuntimeUpgrade for DetectDanglingStorage {
  fn on_runtime_upgrade() { noop } 
  fn post_update() { 
    // iterate over all keys, match them to pallet storage from metadata
    // using the same tools that make `DecodeEntireState`
    // detect all keys that have value and
    // 1. not part of any pallet
    // 2. not part of `Exceptions`
  }

[ggwpez]

Yea ^ that sounds pretty nice.