[media] em28xx: remove unused a8293 SEC config

Devices that were using a8293 SEC are converted to I2C platform data
thus that old config structure is left unused.

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