refactor: remove custom serde derivation for DID types

[wischli]

Connected to https://github.com/KILTprotocol/ticket/issues/2235

• Removes custom (de-)serialization for DID types

On a note: scale-info has a feature flag serde which automatically adds serde derivation to the corresponding type. Thus, we don’t have to add our own custom serde derives for those types anymore.

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)

[ntn-x2]

Nice job 🧹🧹🧹