media: igorplugusb: remove superfluous usb_unlink_urb()

Calling that on yourself while the completion handler
is running is a NOP. Remove it.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>