Remove menpofit widgets

Following the changes introduced by #499 , this PR removes the widgets that are moved to `menpofit`. These are: `visualize_shape_model`, `visualize_appearance_model`, `visualize_aam`, `visualize_atm`, `visualize_fitting_results` and `plot_ced`.

The widgets left in `menpo` are `visualize_images` and `visualize_shapes`.