Fix for #6969 - Filters on restored columns

Tested this code locally (I was having the same issue as the original poster for issue #6969). It works like a charm.

packages/core/src/js/directives/ui-grid-filter.js

@@ -26,6 +26,7 @@
             };
 
             $scope.$on( '$destroy', function() {
+              delete $scope.col.filterable;
               delete $scope.col.updateFilters;
             });
           },