i2c: i2c-smbus: drop useless stubs

Drivers which use the SMBus extensions select I2C_SMBUS, so the
stubs are not needed.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>