Remove monochrome font preferences icon

Summary:
Removes the monochrome font preferences icon to prevent showing it instead of the colorful icon in the system settings "Fonts" category tooltip.

Test Plan: Open the system settings and hover over the "Fonts" category.

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, ndavis, kde-frameworks-devel, VDG

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18702