After commit 68bb0ec22a the shortcut for the Delete action is not
necessarily Shift+Del, but whatever the user set in System Setting.
However DolphinRemoveAction assumes/hardcodes Shift+Del, so we should
always make sure we have this shortcut around, for consistency.
We just need to append it (if necessary) to the list of shortcuts of the
action. (Note that we *don't* prepend it, otherwise it would override a
custom primary shortcut in the 'Configure Shortcuts' dialog).