Brassaia: Remove the metadata type

What

This MR removes occurrences of the abstract metadata type

Why

By replacing any occurrence of metadata by unit

How

This MR removes all the metadata occurrences and makes sure that the types that previously contained a metadata field are serialised/hashed with this absent type in mind since it would break the computed hashes and the number of bytes needed to represent them

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR