window: Remove stack from breakpoint

The stack widget is always visible, and does not need to be
set as visible again when the breakpoint is applied/unapplied.

Part-of: <https://gitlab.gnome.org/GNOME/Incubator/loupe/-/merge_requests/228>

data/gtk/window.ui

@@ -72,7 +72,6 @@
       <object class="AdwBreakpoint">
         <condition>max-width: 590px</condition>
         <setter object="properties_flap" property="orientation">vertical</setter>
-        <setter object="stack" property="visible">true</setter>
       </object>
     </child>
     <property name="content">