'acceptWidgets: true' generates duplicate 'ui-resizable-handle' fix

removed duplicate resize handlers when draging from a grid and dropping onto another - preventing dropped grid from resizing afterward.

Fix originated from  #435 by eagoo

Bug Number: 718
Testing Done: two.hml

dist/gridstack.js

@@ -889,6 +889,7 @@
                     node.el = el;
                     self.placeholder.hide();
                     Utils.removePositioningStyles(el);
+                    el.find('div.ui-resizable-handle').remove();
 
                     el
                         .attr('data-gs-x', node.x)

src/gridstack.js

@@ -889,6 +889,7 @@
                     node.el = el;
                     self.placeholder.hide();
                     Utils.removePositioningStyles(el);
+                    el.find('div.ui-resizable-handle').remove();
 
                     el
                         .attr('data-gs-x', node.x)