applets/devicenotifier: Show "remove all" when only one disk mounted

Currently the "Remove all" item appears only when multiple external disks mounted.
It should be available even if there's only one disk mounted, for the following
reasons:

1. I'm getting used to use "Remove all" to unmount all external disks, but often
   frustrated to not finding it when there's only one disk.
2. Generally, "Close all" actions are available when there's a single thing to
   close. E.g. "Close all files" in text editors (Kate). "Close all tabs" in web
   browsers. 

BUG: 460220
FIXED-IN: 5.27