add ability to unregister a preset

fix  #7218

IPython/html/static/notebook/js/celltoolbar.js

@@ -190,6 +190,22 b' define(['
         }
     };
 
+    /**
+     * unregister the selected preset, 
+     *
+     * return true if preset successfully unregistered
+     * false otherwise
+     *
+     **/
+    CellToolbar.unregister_preset = function(name){
+        if(CellToolbar._presets[name]){
+            delete CellToolbar._presets[name];
+            events.trigger('unregistered_preset.CellToolbar', {name: name});
+            return true
+        }
+        return false
+    }
+
 
     /**
      * List the names of the presets that are currently registered.

IPython/html/static/notebook/js/maintoolbar.js

@@ -134,10 +134,19 b' define(['
             var name = data.name;
             select.append($('<option/>').attr('value', name).text(name));
         });
+        this.events.on('unregistered_preset.CellToolbar', function (event, data) {
+            if (select.val() === data.name){
+                select.val('');
+                celltoolbar.CellToolbar.global_hide();
+                delete that.notebook.metadata.celltoolbar;
+            }
+            select.find("option[value='"+name+"']" ).remove();
+        });
         // Update select value when a preset is activated.
         this.events.on('preset_activated.CellToolbar', function (event, data) {
-            if (select.val() !== data.name)
+            if (select.val() !== data.name){
                 select.val(data.name);
+            }
         });
 
         var wrapper = $('<div/>').addClass('btn-group');