Nairobi: Disable nairobi mempool plugin

What

Part of the freezing process of Nairobi. See https://gitlab.com/tezos/tezos/-/issues/6952

• Depends on !12043 (merged)
• Next !12058 (merged)

This merge request disable the plugin of the mempool for Nairpobi

Why

As Oxford was activated, Nairobi is no longer used

How

See https://gitlab.com/tezos/tezos/-/issues/6952

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