[Cashtab] [BCH deprecation] Pt 2/2 - Deprecate BCH.Address.isCashAddress from cashMethods

Summary:
T2370

Depends on D12387

This diff deprecates BCH.Address.isCashAddress in cashMethods

Test Plan:
npm test and ensure unit test passes
npm start
send a simple XEC tx that results in a change output and observe via the explorer the change is being sent back to the correct change address
grep -r 'BCH.Address.isCashAddress' src/ and ensure no further usage of the bch-js function

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12388