gitlab CI: dnf remove gtk4-devel for the no-debug-gui deps job

We've long preferred GTK4 and that's installed on our images, so let's
make sure that gets removed together with GTK3 (which isn't actually
installed anyway).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
69 jobs for wip/wayland-build-fix in 40 minutes and 56 seconds (queued for 19 seconds)