[GTK3] Revert checkbox recolouring

Summary:
This reverts the effects of commit 7b3d0777 and some effects of commit 987f750e,
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: {F7675941}

Reviewers: #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: ngraham, plasma-devel

Tags: #plasma

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