Proto/Mempool: filtering of non-minimal slots is no longer behind feature flag

What

The mempool previously filtered out non-minimal slots only when the aggregate_attestation feature flag was enabled. This behavior is now part of the upcoming protocol and no longer gated behind the feature flag.

Why

Removing the feature flag gate was requested to simplify the development of other features.

How

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