rm wriet dmat in arap example

Former-commit-id: 2015f3c4778084f2b248ee5218f01546148488ac

examples/arap/example.cpp

@@ -188,7 +188,6 @@ bool harmonic(
     }
     W.col(w) = Ww;
   }
-  writeDMAT("W.dmat",W);
   return true;
 }