Revert "patch deletion in codemirror"

This reverts commit 331a5f7fe85a6e894c35b64cd7987ed53f59ea57.
monkey patch codemirror with new functionality

use marijnh recommended way:
https://github.com/marijnh/CodeMirror/pull/661
update cm update instruction
Merge pull request #2906 from Carreau/mkypatch-cm

Unpatch/Monkey patch CM