[Cashtab] Implement address validation from ecashaddrjs lib

Summary:
Remove all Cashtab functions and unit tests associated with address validation. These are handled at the library level.

An obsolete function with some crossover to address validation (cashMethods/getChangeAddressFromInputUtxos) is also removed (discovered while hunting down remaining addr validation functions).

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15204