Check that there are editing controls when removing tipsy

Fixes the following scenario:
* Open proofreading.
* Edit a message and save it. Wait for it to be marked as
  self-translated.
* Try editing it again. Currently this causes a JavaScript error,
  because tipsy cannot be removed from a non-existent element.

Change-Id: Icbddb735ea505d3c65811c06cf54df7e0cde85e8