Context

This MR removes the legacy mempool that supports Kathmandu. Indeed, it is no longer useful now that Kathmandu is not active on Mainnet anymore.

Checklist

[N/A] Document the interface of any function added or modified (see the coding guidelines)
[N/A] Document any change to the user interface, including configuration parameters (see node configuration)
[nothing new] Provide automatic testing (see the testing guide).
[N/A] 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).
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Edited Jan 18, 2023 by Diane Gallois-Wong