Remove duplicated 'close' button from titlebar of gnome applications

Dom0 enforce decorations which already contain close button.

Thanks @dzklaim for the solution.
Fixes QubesOS/qubes-issues#2813

Makefile

@@ -198,6 +198,7 @@ install-common:
 		misc/20_org.gnome.settings-daemon.plugins.updates.qubes.gschema.override \
 		misc/20_org.gnome.nautilus.qubes.gschema.override \
 		misc/20_org.mate.NotificationDaemon.qubes.gschema.override \
+		misc/20_org.gnome.desktop.wm.preferences.qubes.gschema.override \
 		$(DESTDIR)/usr/share/glib-2.0/schemas/
 	install -g user -m 2775 -d $(DESTDIR)/var/lib/qubes/dom0-updates
 	install -D -m 0644 misc/qubes-master-key.asc $(DESTDIR)/usr/share/qubes/qubes-master-key.asc

misc/20_org.gnome.desktop.wm.preferences.qubes.gschema.override

@@ -0,0 +1,2 @@
+[org.gnome.desktop.wm.preferences]
+button-layout='appmenu:'

rpm_spec/core-vm.spec

@@ -523,6 +523,7 @@ rm -f %{name}-%{version}
 /usr/share/glib-2.0/schemas/20_org.gnome.settings-daemon.plugins.updates.qubes.gschema.override
 /usr/share/glib-2.0/schemas/20_org.gnome.nautilus.qubes.gschema.override
 /usr/share/glib-2.0/schemas/20_org.mate.NotificationDaemon.qubes.gschema.override
+/usr/share/glib-2.0/schemas/20_org.gnome.desktop.wm.preferences.qubes.gschema.override
 /usr/share/nautilus-python/extensions/qvm_copy_nautilus.py*
 /usr/share/nautilus-python/extensions/qvm_move_nautilus.py*
 /usr/share/nautilus-python/extensions/qvm_dvm_nautilus.py*