media: i2c: adv7511: remove unused struct 'i2c_reg_value'

'i2c_reg_value' is unused since the original
commit 5a544cce ("[media] adv7511: add new video encoder").

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>