Cleaning unused Material Slots in Blender 2.80 Beta

Question

In Blender 2.80, I've deleted a material and have empty material slots on my objects. I want to clean this up.

I have this bit of code:

def clean_mat_slots(self):
    bpy.ops.object.select_all(action='DESELECT')

    for ob in bpy.data.objects:
       if ob.type != 'MESH':
           continue

       ob.select_set(state=True)
       bpy.context.view_layer.objects.active = ob

       # Delete all material slots in bounds object
       for i in range(len(ob.material_slots)):
           print("mat slot:", i)
           ob.active_material_index = i - 1
           bpy.ops.object.material_slot_remove()

But it does not clean up the slots as I expect. There are no errors reported when I run the code.

Any ideas?