Proto: minor encodings refactoring

What

apply_results.ml:

• Removed an unused parameter from the consensus_result_encoding definition.

operation_repr.ml:

• Removed a duplicate encoding for consensus_content type.
• Eliminated redundant field-by-field handling in projection and injection functions.

Why

How

Manually testing the MR

Encoding breakings changes should be catched by CI katai checks.

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