Proto/operation_repr: remove DAL content from signed bytes of BLS attestations

What

Under the aggregate_attestation feature flag, for attestations from delegates with a BLS consensus keys, the signed bytes no longer take the dal_content into account.

Why

Because for these attestations, the dal_content will instead be encoded into which public key is used for signing in !17642 (merged).

How

Update the attestation encoding of Operation_repr.Bls_mode to ignore dal_content.

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