Removed redundant checks for item.editable in timeline (#2256)

@simo9000 Thanks!

lib/timeline/component/ItemSet.js

@@ -1211,8 +1211,7 @@ ItemSet.prototype._onDragStart = function (event) {
   if (item && (item.selected || this.options.itemsAlwaysDraggable)) {
 
     if (!this.options.editable.updateTime &&
-        !this.options.editable.updateGroup &&
-        !item.editable) {
+        !this.options.editable.updateGroup) {
       return;
     }
 

lib/timeline/component/item/Item.js

@@ -145,8 +145,7 @@ Item.prototype.repositionY = function() {
  * @protected
  */
 Item.prototype._repaintDeleteButton = function (anchor) {
-  var editable = (this.options.editable.remove || 
-                  this.data.editable === true) &&
+  var editable = this.options.editable.remove &&
                  this.data.editable !== false;
 
   if (this.selected && editable && !this.dom.deleteButton) {