Proto/Michelson: remove most field_annot

Context

Fourth step to ignoring annotations (tzip!138 (closed), #2025 (closed)): getting rid of field annotations for comparable pairs and unions and regular pairs but not unions yet because of entrypoints (this is a redo of the fourth part of !2482 (closed)). The third step was !4141 (merged).

The first 4 commits remove field_annot from comparable_ty, the 4 following remove field_annot from Pair_t, the others are just consequential simplifications.

Tests

See updated regtests

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, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR