This reverts commit 7c504473b0.
We need to revert this CodeMirror upgrade because it broke displaying
the file diff for certain files as reported by issue 3927 [1].
[1] https://code.google.com/p/gerrit/issues/detail?id=3927
Bug: issue 3927
Change-Id: Id6ee70e3a8e9b5e5e31cde9bddc09106088ff31d