Topicmaps: unregister viewmodel customizer (#505).

View Customization Framework:

The Topicmaps API has a method to unregister a viewmodel customizer:
{{{
void unregisterViewmodelCustomizer(ViewmodelCustomizer customizer)
}}}

A viewmodel customizer author should call this method once the Topicmaps service goes away. Otherwise hers plugin might have problems when redeployed.

See #505.