[Cashtab] [Simplify wallet structure p3] Remove unused utxos array

Summary:
T2663

Depends on D12592

Remove the unused utxos array from wallet state.

Because isValidStoredWallet must recognize the wallets from before this stack and after this stack as valid, adjust unit tests and mocks accordingly.

Test Plan:
Review unit test adjustments and npm test
npm start
Wait for update to run
Change wallets
Wait for update to run
Observe local storage and see that wallet state is updated for wallet and the previously active wallet in savedWallets

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12593