[Cashtab] [p1 local msg verify] Verify function is not async

Summary:
T2730

Stacked diff to deprecate bchObj.BitcoinCash.verifyMessage

This diff removes the async from the existing function, as bchObj.BitcoinCash.verifyMessage is not an async function. This is a bug fix as preparation for dropping in a replacement function.

Test Plan:
npm start
Verify some messages, note no change in existing functionality

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12458