LSPS1: Disallow compiler warnings once implementation is finished

In https://github.com/lightningdevkit/lightning-liquidity/pull/86 we denied warnings in CI generally, but allowed them explicitly for LSPS1 as the implementation is not finished and currently hidden behind a `cfg` flag anyways.

We should however make sure to disallow warnings again, fix any remaining ones before we drop the `cfg` flag.