MC-3500: TinyMCE toolbar doesn't overflow the neighbor content type

- Removed z-index causing layering issues

Author: nathanjosiah

app/code/Magento/PageBuilder/view/adminhtml/web/css/source/content-type/_preview.less

@@ -64,7 +64,6 @@
             text-align: center;
             transition: .3s;
             width: 100%;
-            z-index: 1;
         }
     }