rm unnecessary include

Former-commit-id: 18ac0083e7995bb41e7551b80552f69803e50a85

include/igl/slice.cpp

@@ -7,7 +7,6 @@
 // obtain one at http://mozilla.org/MPL/2.0/.
 #include "slice.h"
 #include "colon.h"
-#include "matlab_format.h"
 
 #include <vector>