Remove the separator in the HeaderBar

Back when there was a Save icon on the Sink page's HeaderBar, it made
sense to keep the separator around in the other pages.  Now there's
never a separator there, so we shouldn't force one to exist in some
pages anymore.

data/pd-buddy-gtk.ui

             <property name="can_focus">False</property>
             <property name="title">Select Device</property>
             <property name="show_close_button">True</property>
-            <child>
-              <object class="GtkLabel" id="dummy">
-                <property name="visible">True</property>
-                <property name="can_focus">False</property>
-              </object>
-              <packing>
-                <property name="pack_type">end</property>
-              </packing>
-            </child>
           </object>
           <packing>
             <property name="name">page0</property>