Remove direct disable of GL_SCISSOR_TEST

The vtkOpenGLState for GL_SCISSOR_TEST could be modified elsewhere. If
it was enabled in the state object, then invoking
glDisable(GL_SCISSOR_TEST) directly prior to setting the value in the
OpenGL state would cause a warning to be thrown with every render
because the cached state no longer reflected the OpenGL
state. Moreover, the direct call was redundant as the vtkOpenGLState
object will make the direct call as needed.