[Cashtab] Deprecate API wrapper - signPkMessage

Summary: As per T2370, deprecating signPkMessage as it was just an API wrapper.

Test Plan: sign a message, note the signature, message and address, then verify this matches via the verify message function.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12261