scimoz: ignore when trying to remove a listener that isn't registered
* otherwise this NP method would return false and throw an exception,
  which gets logged into the console (even though it was handled in JS)

(integrated from komodo (trunk) change 84934 by toddw)

git-svn-id: http://svn.openkomodo.com/repos/openkomodo/trunk@14344 b87d16e7-d29f-4ee1-b5ec-817d645d335f