Remove inexistent hook "LivePreviewPrepare"

This hook is never fired, therefore, per code review of
Ife952fc165dacea3ae7726e09bf61279f4bdae59 it should be
removed from CodeEditor.

Bug: 47235
Bug: 52071
Change-Id: Ic9dd9066f7d1676f45381cb6796425f13490fea3