Unconditionally accept Simplicity spends in the mempool

[roconnor-blockstream]

Taproot and other soft-forks historically in Bitcoin have not gated mempool access.

[roconnor-blockstream]

@apoelstra It would be nice to test your hal-simplicity workflow against this PR.

[roconnor-blockstream]

This PR is optional, but after reviewing bitcoin/bitcoin#19953 I see that historical Bitcoin activation of similar soft-forks have no mempool logic, and simply allow the new transaction type unconditionally in the next version of the software. Instead they relying on users not to use create such transaction until after activation (or risk having their funds stolen).

To than end this PR slightly simplifies our codebase and bring somewhat more in line with Bitcoin's.

This is a policy change and not a consensus rule change.

[delta1]

utACK ddda11c

Simplicity will already be active on liquidv1 by the time this is released.

[apoelstra]

utACK ddda11c

[roconnor-blockstream]

I strongly recommend not merging it until there is a tested ack.

[delta1]

ACK ddda11c

Ran tests locally

[roconnor-blockstream]

I still suggest that @apoelstra test his hal-simplicity work against this PR.