Fix broken test, test_ode.py::test_lie_group

test_lie_group utilized an incorrect simplification that was fixed PR 7641.

For details see discussions @:

https://github.com/sympy/sympy/pull/7674
https://github.com/sympy/sympy/pull/7641
https://github.com/sympy/sympy/pull/7638
https://groups.google.com/forum/#!topic/sympy/p3hq7MCY34o