Proto/Michelson: remove metadata from basic types

Context

First part of the sixth step to ignoring annotations (tzip!138 (closed), #2025 (closed)): getting rid of metadata for basic types, this is just a big simplification. This part was not strictly in the original MR !2482 (closed) because metadata didn't exist at that time. The fifth step was !4259 (merged).

The first commit should be moved to the second part of the sixth step but I don't want to solve conflicts. It doesn't hurt keeping it here.

Fixes #2374 (closed)

Testing

CI

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).
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