Remove delete icon in toolbar for sys_file_metadata
If an editor opens a file in the filelist by clicking on the title, the
sys_file_metadata record is opened. An editor expects nevertheless, that the delete button in the header bar will delete the file itself.
it doesn't make sense to show the delete icon as the record itself will be recreated on the fly automatically when the file is fetched, e.g. reload of the filelist module.