[Cashtab] [Simplify wallet structure p4] Remove parent artifact object slpBalancesAndUtxos

Summary:
T2663

Depends on D12593

Remove parent object slpBalancesAndUtxos from wallet structure

Note: this means that this wallet structure should not longer be supported by any functions in Cashtab...with the exceptions of isValidStoredWallet (because stored wallets may be in the old format) and loadStoredWallet (for the same reason)

Hence, this diff modifies mocks in existing unit tests.

Test Plan:
Review unit test mock changes and npm test
npm start
Wait to see dev console msg that active wallet has been updated. Confirm by reviewing local storage.
Change wallets. Wait to see dev console msg that active wallet has been updated. Confirm by reviewing local storage.
Switch back to original wallet. Confirm both used wallets are stored correctly in local storage.
Send an XEC tx
Create a token
Burn a token
Send an eToken tx

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D12625