Contour Module: unregister active representation
Contour Module: unregister active representation

When I cleaned up the contour module in commit
dcd0021a88d6d3b7a94bf77f612cb36463d321cc, I forgot
to unregister the active representation as well.

OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,mathturtle/tomviz,OpenChemistry/tomviz,OpenChemistry/tomviz,mathturtle/tomviz