alpha: remove balance_updates_encoding_with_legacy_attestation_name encoding

What

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

This MR removes deprecated balance_updates_encoding_with_legacy_attestation_name encoding in the protocol code.

This MR is on top of !13451 (merged)

Why

How

Remove balance_updates_encoding_with_legacy_attestation_name encoding in the protocol code.

Manually testing the MR

Since the changes are only protocol internal and the specific test for legacy encoding has been disable the only check is the CI green.

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