[wmlunits] clear <config>/data/add-ons

Sometimes if the last run aborted in the wrong moment, add-ons may keep
left overs in there.

data/tools/unit_tree/update-wmlunits

@@ -25,6 +25,7 @@ BUILD=$DIR/build
 TOOLS=${TOOLS:-$SOURCES/master/data/tools}
 export TEMP=$DIR/temp
 
+rm -rf $CONFIG/data/add-ons
 mkdir -p $CONFIG/data/add-ons
 mkdir -p $ADDONS
 mkdir -p $LOG