All linux CI machines have gtk3 installed. And Python GTK things
are too complicate to setup.
Change-Id: I3c0d967f61aebea508784df79569b9d0064f66e2
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit aef55501bccd52f63a84228cb2ee201ec4f5c301)
Reviewed-by: Liang Qi <liang.qi@qt.io>