The Legacy charts store their information in multiple places in the tree.
The actual data is stored on a StatisticsObject stored in MODEL:/Tools/Statistics.

The chart then reference that Statistics object. Any added performance measures also reference that Statistics object.
If you delete the chart and you delete the performance measure, it will automatically delete the associated Statistics object once nothing is using it.
The list of potential performance measures that is displayed is based on the Statistics objects that are in your model. So after deleting the chart and the performance measure, the Statistics object should also get deleted. Then when you close and reopen the Experimenter window, they should no longer be in the list of performance measures that you can add to the experiment.
You can also manually delete the Statistics objects in the tree, but that might cause problems if you still have other objects that reference those objects.
Phil BoBo
Sr. Manager, Software Development