The ProofPool container stores a proof reference but index them by their proof id. Having removeProof() take the proof ref makes it unclear that we are removing a proof with the same id and not a proof reference with the same underlying pointer, so this diff updates the API to take a proof id instead.

The proof id is still passed by value, as the common use case is to get a reference from proof->getId(). Since there might be several entries for that proof id (1 per utxo), the proof might be deleted and the id reference dangling as the erasure loop is processed.

Ref T1854.