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
Now we have broken test:
https://s3.amazonaws.com/archive.travis-ci.org/jobs/38145753/log.txt

It seems, we have is_diagonalizable tests for symbolic matrices.  So, it's clear - something *was* implemented to support them and this comment should be removed.