vtkOpenGLContextActor should subclass vtkContextActor

This was true for the OpenGL2 variant, but not OpenGL. This caused
the IO/Export tests to fail as they could not safe down cast the
actor in the prop collection. Also removed unneeded includes etc
from the parent class.

Change-Id: I1acd77e84df3d4a1362223bbc1581b626731c3da