i2c: remove check that can never be true

A driver remove callback is only called if the device was bound before.
So it's sure that both dev and dev->driver are valid and dev is an i2c
device. If the check fails something louder than "return 0" might be
appropriate because the problem is grave (something like memory
corruption), otherwise the check is useless.

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