[Cashtab] [Deprecate BCH msg signing p1/3] Correct async bug

Summary:
T2730

Now that we have a function to get EC Pair from Wif, we can also deprecate bchObj.BitcoinCash.signMessageWithPrivKey

This diff patches a bug in that this function is treated async but it is not

Test Plan:
Review here to confirm the function is not async: https://github.com/Permissionless-Software-Foundation/bch-js/blob/master/src/bitcoincash.js#L150
npm start
sign and verify a msg

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12626