[electrum] exclude already added utxos when adding more wallet UTXOs to a proof

Summary:
When adding additional stakes from the wallet to an existing proof, it is confusing to have to choose from a list that contains already added stakes.
Filter the list accordingly.

Test Plan: Select a few coins, right click, Build Avalanche Proof, click "add coins from wallet" button and check that the list now no longer has the coins already in the proof.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18770