Revert merge pull request #528

This pull request reverts the merge of #528 ("Add savePNGFile function for histograms").

A couple of problems with that pull request have shown up:

* It introduced a dependency on _vtk_ for all users of 'png_io.h', e.g. Kinfu Large Scale (#640, #645, [pcl-users](http://www.pcl-users.org/Kinfu-Large-Scale-Compile-error-vtkPNGWriter-h-not-found-td4033644.html));
* It is not compatible with _vtk 6_ (#363).

I think that if we are to keep this functionality in PCL, we need to a) update the code that it is compatible with _vtk 6_; b) find a better place for it so that the users concerned with actual PNG _input/output_ do not get unnecessary dependencies.