[Cashtab] [Simplify wallet structure p2] Remove duplicate tokens array

Summary:
T2663

Depends on D12590

Remove repeated tokens object. This is used nowhere in the app and can simply be removed.

Test Plan:
npm start
Wait for update to run at least once and to see dev log msg confirming updated wallet has been saved to storage
Change wallets
Wait for update to run at least once and to see dev log msg confirming updated wallet has been saved to storage
Change back
Review local storage. Click 'update' to refresh local storage display. Confirm that the wallet you used is saved without the duplicate tokens array under savedWallets

grep -r slpBalancesAndUtxos.tokens src/ has no output
Review grep -r slpBalancesAndUtxos to confirm these references are not using the slpBalancesAndUtxos.tokens array

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D12592