include/linux/i2c.h: remove I2C_CLASS_DDC support

After removal of the legacy EEPROM driver and I2C_CLASS_DDC support in
olpc_dcon there's no i2c client driver left supporting I2C_CLASS_DDC.
Class-based device auto-detection is a legacy mechanism and shouldn't
be used in new code. So we can remove this class completely now.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>