V4L/DVB (4389): Remove duplication _release() op.

I added a duplicate method during one of the previous dvb_attach attempts.
This removes the unnecessary duplication.

Signed-off-by: Andrew de Quincey <adq_dvb@lidskialf.net>
Acked-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>