[GTK3] Revert checkbox recolouring

Summary:
This reverts the effects of commit 7b3d0777ea6c10dd430ed686434d3a4e2d072632 and some effects of commit 987f750ea16a6f82d29cd6504a69d93575c895d4,
meaning that checkboxes and radiobuttons will no longer attempt to follow the colourscheme.

This is necessary because the facilities used to recolour checkboxes are now glitchy in GTK
and result in usability issues. See https://gitlab.gnome.org/GNOME/gtk/issues/2217

BUG: 412078
FIXED-IN: 5.17.2

Test Plan:
Before: https://bugs.kde.org/show_bug.cgi?id=412078
After:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: ngraham, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24994