claimpegin optional `claim_script` argument is pretty much useless

As-is, if the wallet doesn't have the `claim_script` address in its address book, it will not be able to sign it regardless, requiring a `createrawpegin` step then pass to the wallet that knows how to sign the input.

The only effect the optional argument has is that it can find a specific peg-in output, if there are multiple with unique output scripts, bypassing the "return first seen" behavior currently in the wallet code. Our wallet doesn't even support multiple outputs per peg-in transaction, so this is never exposed to the user.