media: dvb-usb: dib0700_devices: Remove unused struct dibx090p_adc

After commit 5e9c85d9("[media] dib8096: enhancement"), no one use struct
dibx090p_adc, so remove it.

Link: https://lore.kernel.org/linux-media/20220927133827.99097-1-yuancan@huawei.com


Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>