Fix a crash when deleting vtk-python objects.

This fixes a bug that has been present since commit 269cbea1 on Oct 16,
2013 where deleting an object that had a DeleteEvent observer would cause
a crash (the crash was only verified to occur on OS X).

Change-Id: I89d7a9a6eff9ab11fdcd7291a42f47b9a8017091
Merge topic 'python-ptr-erase' into master

cbd4884d Fix a crash when deleting vtk-python objects.