[Cashtab] [p8 local msg verify] Deprecate legacy verification function

Summary:
T2730

Depends on D12464

Remove the legacy approach at signature verification

There is an unrelated bug where the message signing function is using BCH instead of bchObj. This would be fixed in another diff, except that we would have a build error in this diff if we left bchObj unused. So, I've patched it here.

Test Plan:
npm start
Sign and verify a message in Cashtab
Confirm cashtab.com also verifies the signature

Sign a message in ElectrumABC using an ecash signature
Verify locally with this diff
Verify that cashtab.com calls it false

Reviewers: emack, #bitcoin_abc

Reviewed By: emack, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12465