[Cashtab] Remove unreachable loadStoredWallet migration helper fn

Summary:
With D15256 landed, we can no longer pass isValidStoredWallet with the type of wallet that would benefit from the limited migration implemented in the confusingly named loadStoredWallet function

Delete this function. If Cashtab loads with an invalid wallet, it will populate wallet state from chronik and not by modifying params in the local storage object.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15243