[Cashtab] [BCH Deprecation] [HD Node] Pt 3/3 - Deprecate BCH.HDNode

Summary:
T2730

Depends on D12447

This is part 3 of the 3 part diff that fully deprecates the use of the BCH.HDNode object.

Pt 1/3 - Implement separate masterHDNode creation logic
Pt 2/3 - Localize downstream logic from masterHDNode
Pt 3/3 - Deprecate BCH.HDNode

Test Plan:
grep -r 'BCH.HDNode' src/ and ensure no hits
npm test
npm start
create new wallet
send and receive XEC with the new wallet's ecash address
send and receive encrypted messages to test the funding WIF extraction logic

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12467