Remove unused Eigen submodule and ortools CMake integration

Author: tipf

.gitmodules

@@ -1,6 +1,3 @@
-[submodule "externals/eigen"]
-	path = externals/eigen
-	url = https://gitlab.com/libeigen/eigen.git
 [submodule "externals/ceres-solver"]
 	path = externals/ceres-solver
 	url = https://ceres-solver.googlesource.com/ceres-solver