rm unneeded define

Former-commit-id: 3dfce85202ce05161a2d53c837c5da33988ca120

include/igl/harwell_boeing.h

@@ -9,7 +9,6 @@
 #define IGL_HARWELL_BOEING_H
 #include "igl_inline.h"
 
-#define EIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET
 #include <Eigen/Sparse>
 #include <vector>