Once created, covariates created with advanced feature "Add Cluster-Based Covariate" are not removed if option deselected.

[jmelott]

To reproduce:
Open sample matrix.
Click next twice to get to Cluster Matrix page.
Select "Show Advanced Features" checkbox.
For both axes, select "Hierarchical Clustering".
For both axes, select "Add Cluster-Based Covariate" checkbox and select some numeric option for "Clusters" dropdown for each.
Click "Apply" button.
(A covariate named "Clusters" should be created on each axis.)
For both axes, deselect the "Add Cluster-Based Covariate" checkbox.
Click "Apply" button.
Clusters created by this should be removed, but remain.