Proto: remove sponsored operations partial implementation from Q

What

Remove Sponsored Operations-related protocol code.

Why

It's not going to be activated in Q in any case, because we are exploring a smart contract-based solution instead. We could keep the code with the feature flag set to false. But it's better to remove this code altogether so that we don't have the extra dead encodings, which might be confusing and would reserve some binary tags forever. This MR can easily be reverted later if we decide on a solution based on this work.

How

• validate.ml is carefully updated to remove everything specific to Sponsored Operations, but keep some refactorings that are useful in general.
• Test helpers are easier to update by hand because they have been modified over multiple commits and sometimes conflict with changes we want to keep.
• Other commits are directly reverted (with small conflicts that are easily handled).

See individual commit descriptions.

Manually testing the MR

• CI
• Words to grep: "sponsor", "guest", "host" (note that "host" has a lot of unrelated hits, and "guest" has some)

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