Get rid of some unnecessary commented out code (issue #6849)

There is a lot of false positives (see [attached logs](http://code.google.com/p/sympy/issues/detail?id=3750#c1)), so I don't think that we should automate such cleanup.

fixes #6849
To be safe I would change this to the get form (after replacing `get(r` with `get(k`) since it is clear today what those defaults should be; if you delete the code and it is ever needed someone will have to find out what the defaults should be.