gui: remove macOS start on login code

Summary:

The macOS startup item code was disabled for builds targeting macOS >
10.11 in #15208. Now that we require macOS 10.12 as a minimum, #17550,
we can remove the startup item code entirely, as the API we were using
was removed in macOS 10.12.

We have OSX_MIN_VERSION=10.12 set in depends/hosts/darwin.mk

This is a backport of PR15208 and PR17567

All code modified in src/qt/guituil.cpp in PR15208 is removed in PR17567, including everything related to *fix memory missmanagement*.

Test Plan:
I do not have a Mac to test this, so I rely on CI cross compilation tests.
ninja all check-all

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8280