Stop exporting DECONFIG_GARDABLE to Serverwiz

ATTR_DECONFIG_GARDABLE is explicitly defined inside the target
definition so there is no reason for SW2 to set it in the system
MRW xmls.

Change-Id: Ia740794fe0bd472475edbc40861e1cfeb7f65e20
Reviewed-on: http://ralgit01.raleigh.ibm.com/gerrit1/46347
Tested-by: Jenkins Server <pfd-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP Build CI <op-jenkins+hostboot@us.ibm.com>
Tested-by: Jenkins OP HW <op-hw-jenkins+hostboot@us.ibm.com>
Tested-by: FSP CI Jenkins <fsp-CI-jenkins+hostboot@us.ibm.com>
Reviewed-by: Christian R. Geddes <crgeddes@us.ibm.com>
Reviewed-by: Martin Gloff <mgloff@us.ibm.com>
Reviewed-by: Corey V. Swenson <cswenson@us.ibm.com>
Reviewed-by: Daniel M. Crowell <dcrowell@us.ibm.com>

src/usr/targeting/common/xmltohb/attribute_types.xml

@@ -582,6 +582,7 @@
         <default>0</default>
       </uint8_t>
     </simpleType>
+    <no_export/>
   </attribute>
 
   <attribute>