Update import_manager.rb

When inferences are rebuilt, triples may be added to the graph named: <http://vitro.mannlib.cornell.edu/default/vitro-kb-inf-rebuild>.

When an entity is removed, we iterate over a collection of graphs that might contain data for that entity and remove all triples that contain its uid as either a subject or object.  We need to add vitro-kb-inf-rebuild to the collection of graphs that are scrubbed of data for the entity.