[Cashtab] [BCH deprecation] Pt 2/2 - Deprecate BCH.Address.toHash160

Summary:
T2730

Depends on D12395

This diff switches out all uses of the BCH-JS toHash160() function to the local implementation.

Test Plan:

• npm test and ensure existing unit tests pass
• grep -r 'BCH.Address.toHash160' src/ and ensure no further use of this external function
• npm start
• send a private message to another address and ensure successful decryption on the receiving wallet
• create a new wallet (to trigger deriveAccount()), then send a private message using the new wallet and ensure successful decryption on the receiving wallet

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12396