Treat not set parameter 'linktitle' as request to remove sitelink

* Remove a sitelink when the 'linktitle' parameter is not set or
  is an empty string

Change-Id: Iaf608a43b1dd820b0fc81f51c17236f00cf7c6e4
Merge "Treat not set parameter 'linktitle' as request to remove sitelink"