Remove obj_c for macOS Dock icon menu

Summary:

Qt `setAsDockMenu()` does this work.

Partial backport of core PR14123 (commit 6b1d297):
https://github.com/bitcoin/bitcoin/pull/14123/commits/6b1d2972bf9a40f97ba3a5c95831fd179b1054cf

Depends on D4480.

Test Plan:
Build and run bitcoin-qt for macOS.
Check the docked icon works as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4481