Remove 1 px border around Start Center

It leaks workspace background color (behind the sheet in Writer etc.), which
looks bad as it's much darker now under Windows and OS X.

Change-Id: I29034be86d3cf5537384b325fc6734b4dc05f077
Reviewed-on: https://gerrit.libreoffice.org/10521Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sfx2/uiconfig/ui/startcenter.ui

@@ -15,7 +15,7 @@
         <property name="can_focus">False</property>
         <property name="hexpand">True</property>
         <property name="vexpand">True</property>
-        <property name="border_width">1</property>
+        <property name="border_width">0</property>
         <child>
           <object class="GtkBox" id="all_buttons_box">
             <property name="visible">True</property>