Remove v0 mempool

[ninabarbakadze]

Description

v0 mempool is currently not enabled, therefore we should remove it from the codebase.

[cmwaters]

It is currently possible for a node operator to run with the v0 mempool. The original reason for having it around was because the priority mempool itself was new and it was good to have a fallback option. It does seem like the priority mempool has been working well so I would be okay with removing v0 for the purpose of reducing the maintenance burden

[rootulp]

Given the implementation is already complete and we're not frequently modifying the v0 implementation, is there a lot of maintenance burden from keeping it?

IMO if we're going to remove support for it, we should deprecate it for some time prior to removing support for it so that node operators see some messaging from us that it will be removed soon and can modify their config to use a different mempool implementation.