[Views] Clear regions on destroy

Clear regions when a view is destroyed. This causes a view's
destroy method to be correctly invoked, allowing views to do
cleanup. Used by NSS heatmap view for sprint Alice,
https://github.com/nasa/openmct/projects/1

src/adapter/directives/MCTView.js

@@ -33,6 +33,7 @@ define([
             link: function (scope, element, attrs) {
                 var region = new Region(element[0]);
                 scope.$watch(attrs.mctView, region.show.bind(region));
+                scope.$on("$destroy", region.clear.bind(region));
             }
         };
     }