Remove obj_c for macOS Dock icon setting

Summary:

Qt `setWindowIcon()` does this work.

Partial backport of core PR14123 (commit 53bb6be):
https://github.com/bitcoin/bitcoin/pull/14123/commits/53bb6be3f8a50ee9e5c4d7e9155236152e7c4b7c

Test Plan:
Build for OSX and run bitcoin-qt.
Check the dock icon shows up and the show/hide feature is working.
Tested on OSX Catalina.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4472