sidebarrow: Remove useless size group

Nice try, but size groups don't work with invisible widgets anyway.
Invisible widgets request 0×0.

[reapplying after accidental reintroduction in the cloudproviders patch;
see https://bugzilla.gnome.org/show_bug.cgi?id=786123#c39]

gtk/ui/gtksidebarrow.ui

@@ -78,14 +78,4 @@
       </object>
     </child>
   </template>
-  <!-- We need it to not make the row smaller when the eject button is hidden -->
-  <object class="GtkSizeGroup">
-    <property name="mode">vertical</property>
-    <widgets>
-      <widget name="eject_button"/>
-      <widget name="label_widget"/>
-      <widget name="start_icon_widget"/>
-      <widget name="end_icon_widget"/>
-    </widgets>
-  </object>
 </interface>