It seems the #define statement is no longer needed.

Former-commit-id: 7822afe52e408a41a3cb297db3032d458f7d18f9

include/igl/slice.h

@@ -9,9 +9,6 @@
 #define IGL_SLICE_H
 #include "igl_inline.h"
 
-#ifndef EIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET
-#define EIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET
-#endif
 #include <Eigen/Sparse>
 namespace igl
 {