Remove texture maps from one or many Materials

I need to remove all texture maps from the bump slot on a selection of objects.
I have tried to specify None type as the bitmap path and a variety of other experiments. I see how to Add a bitmap to a material in the documentation, but can’t find a method to remove.

import rhinoscriptsyntax as rs
import scriptcontext as sc

obs = rs.GetObjects("Pick")

for obj in obs:
    index = rs.ObjectMaterialIndex(obj)
    if index >-1:
        #rs.MaterialBump(index, None)
        mat = sc.doc.Materials[index].RenderMaterial