DevTools: [CodeMirror] do not leak editor through settings listeners

This patch removes settings change listeners to avoid leaks of
editor instances.

BUG=371218

Review URL: https://codereview.chromium.org/282753005

git-svn-id: svn://svn.chromium.org/blink/trunk@174009 bbb929c8-8fbe-4397-9dbb-9b2b20218538