Further update for CVE-2019-16774/kopano-webapp-plugin-files

5c047a14 ("Update information for
CVE-2019-16774/kopano-webapp-plugin-files") moved the status to
'unfixed' as confirmed by Carsten Schoenert. But further update from
upstream confirms that the cookie backend is not used in the
files-plugin.

Mark the issue as unimportant for kopano-webapp-plugin-files.

Thanks: Carsten Schoenert