HID: usbhid: remove unused hiddev_driver

Now that hiddev_driver isn't being used for anything, there's no
reason to keep it around.  This patch (as1419) gets rid of it
entirely.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>