Revert "Make the kde-gtk-config kcm better at checking global gtk settings"

Summary:
This reverts commit 34357f74ee2d98128ff423b0ec6ddcbf4232c475.

Reverting this fixes loading of the actually used GTK settings.

BUG: 382291

Test Plan:
Opened kcmshell5 kde-gtk-config with and without this revert.
Without, it shows Adwaita as theme, with it shows breeze.
GTK uses breeze, so the behaviour with the revert is correct.

Reviewers: Plasma, apol

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D8443