[backport#16969] refactor: Remove Qt function to disable menu icons on macOS

Summary:
3eea6a8f2686352a0fd868fee6af42ef1283bdda refactor: Remove Qt function to disable menu icons on macOS (Emil Engler)

Pull request description:

As menu icons were removed in #16612, this removes an unnecessary function for macOS
Could this get into v0.19.0?

Backport of Core PR16969

Depends on D8749

Test Plan:

ninja all

Unfortunately I have no macOS to test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8750