[Cashtab] Deprecate unused function

Summary:
T2679

This function is not doing anything. A <div> doesn't have an "onCopy" handler.

Test Plan:
npm test

npm start and copying address works as normal

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14513