Remove -DEIGEN_YES_I_KNOW_SPARSE_MODULE_IS_NOT_STABLE_YET

This has been removed from Eigen 3.1.0-alpha1 (released in 2011).
http://eigen.tuxfamily.org/index.php?title=ChangeLog#Eigen_3.1.0-alpha1
https://bitbucket.org/eigen/eigen/commits/c13088f10146