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
On Wed, Oct 01, 2014 at 07:40:02AM -0700, Christopher Smith wrote:
>    In sympy/core/numbers.py:
> 
>  > @@ -678,9 +677,6 @@ def num(self):
>  >
>  >      def _as_mpf_val(self, prec):
>  >          rv = mpf_norm(self._mpf_, prec)
>  > -        # uncomment to see failures
>  > -        #if rv != self._mpf_ and self._prec == prec:
>  > -        #    print self._mpf_, rv
> 
>    In case it's hard to believe that you have to renorm self you can see that
>    it is necessary. In retrospect, the better place to handle this is ...

The better (worse: the right!) place for this is the debugger.  Set the breakpoint,
set the breakpoint condition...  Pdb can do all this.