[media] rtl28xxu: remove unused SDR attach logic

That logic was duplicated from rtl2832_sdr.h in order to avoid hard
dependency for staging directory. rtl2832_sdr is moved to media, so
we could remove that code now.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>