[media] au0828: remove unused macro

An V4L2_CID_PRIVATE_SHARPNESS macro is defined in the au0828 driver, but
never used. Remove it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>