Remove OK button in About dialog

Doing so fixes a Gtk warning:
"Content added to the action area of a dialog using header bars"

Author: Tilka

dwarview.glade

@@ -54,19 +54,7 @@ Author: Namhyung Kim <[email protected]>
             <property name="can_focus">False</property>
             <property name="layout_style">end</property>
             <child>
-              <object class="GtkButton" id="about_ok">
-                <property name="label">gtk-ok</property>
-                <property name="visible">True</property>
-                <property name="can_focus">True</property>
-                <property name="receives_default">True</property>
-                <property name="use_stock">True</property>
-                <signal name="clicked" handler="gtk_widget_hide" object="about" swapped="yes"/>
-              </object>
-              <packing>
-                <property name="expand">True</property>
-                <property name="fill">True</property>
-                <property name="position">0</property>
-              </packing>
+              <placeholder/>
             </child>
           </object>
           <packing>