refactoring solver_cese.py: abandon variable dtx
refactoring solver_cese.py: abandon variable dtx

solvcon/solvcon,solvcon/solvcon,yungyuc/solvcon,yungyuc/solvcon,yungyuc/solvcon,solvcon/solvcon,yungyuc/solvcon,solvcon/solvcon,solvcon/solvcon,yungyuc/solvcon,solvcon/solvcon,yungyuc/solvcon