i2c: dummy: Drop no-op remove function

A remove callback that just returns 0 is equivalent to no callback at all
as can be seen in i2c_device_remove(). So simplify accordingly.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Wolfram Sang <wsa@kernel.org>