Fixed event handler management in wikibase.client.linkitem.init

When closing the dialog, all event handlers are removed from the link item link now,
preventing the "close" handler from being called multiple times when opening/closing
the dialog multiple times.

Change-Id: I7450a9548e8e01c0f6661ddab1770ba0a8896d3e
Merge "Fixed event handler management in wikibase.client.linkitem.init"