Proto: fix ignored values (part 1/3)

Context

It is always dangerous to ignore values. They may be:

• partial applications that we wanted to be fully applied,
• monadic values that should have been bound,
• states that should be handled linearly (e.g. the context, see the last commit).

This MR removes the pattern let[^ ]* _[^ ]* = for lib_protocol (excluding tests, done in !6579 (merged)). !6583 (merged) takes care of binding operators.

To follow-up: make a linter.

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