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
Are you +1 for the rest (i.e. except [ascii-stuff](https://github.com/sympy/sympy/pull/2219#discussion_r18987949))?