[CMake] Removed option OGS_USE_EIGEN.

CMakeLists.txt

MathLib/CMakeLists.txt

MathLib/LinAlg/ApplyKnownSolution.h

MathLib/LinAlg/GlobalMatrixVectorTypes.h

MathLib/LinAlg/LinAlg.cpp

MathLib/LinAlg/LinAlg.h

MathLib/LinAlg/MatrixVectorTraits.cpp

MathLib/LinAlg/MatrixVectorTraits.h

MathLib/LinAlg/SetMatrixSparsity.h

MathLib/LinAlg/UnifiedMatrixSetters.cpp

MathLib/LinAlg/UnifiedMatrixSetters.h

Tests/MathLib/TestGlobalMatrixInterface.cpp

Tests/MathLib/TestGlobalVectorInterface.cpp

Tests/MathLib/TestLinearSolver.cpp

scripts/cmake/test/AddTest.cmake

scripts/cmake/test/MeshTest.cmake