alpha/client: remove operation encoding supporting legacy attestation name

What

The time to put the legacy attestation name endorsement to sleep has come. This MR is part of a set that target to remove the support for this old name.

This MR removes encoding in the client that support the legacy attestation name endorsement.

This MR fixes #5528 (closed).

Why

How

Remove the operation_with_legacy_attestation_name and unsigned_operation_with_legacy_attestation_name in the client.

Manually testing the MR

CI green. You can also ensure that the octez-codec does not know anymore these encodings for protocol Alpha.

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