Proto/Michelson: no stack var annot

Context

First step to ignoring annotations (tzip!138 (closed), #2025 (closed)): getting rid of stack variable annotations (this is a redo of the first part of !2482 (closed)).

The first commit does remove the var_annot from the Item_t constructor of stack_ty, the others are just consequential simplifications.

Tests

• See regression tests outputs, thanks to #1088 (closed)
• CI, I'll probably have to update some tests

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