revert border styling perf change and merge official fix from CodeMirror upstream master

Author: jasonsanjose

src/styles/brackets_codemirror_override.less

@@ -106,12 +106,7 @@
 
     .CodeMirror-gutters {
         background-color: @background-color-3;
-        
-        /*
-        Maintain line gutter border-right to avoid repaint performance issues
-        See https://github.com/marijnh/CodeMirror/issues/1514
-        */
-        border-color: @background-color-3;
+        border-right: none;
     }
     .CodeMirror-linenumber {
         color: @accent-comment;