Polyline cleaning

This addresses #350

Background:
Since we don't support all KA exercises, we currently remove them from the topic tree.  When we remove all exercises of a topic with no videos, we remove the topic.

Issue:
While we deleted topics, we didn't update the knowledge map to properly remove polylines of deleted topics.

Fix:
Do that!

Testing:
Pull, load up the topic tree, click around!  Note issue #275