[Cashtab] [p4 local msg verify] Deprecate segwit support from xecjs-message verify

Summary:
T2730

Depends on D12460

Remove segwit support from the verify function of xecjs-message, since this is irrelevant to any xec application

In the future, may wish to remove segwit support from all of xecjs-message. However this is beyond the scope of this individual task.

Test Plan:

cd web/cashtab
npm start

Sign and verify msgs, confirm still works

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D12461