of: unittest: Drop call to platform_set_drvdata(..., NULL)

The driver core already unsets driver data after .remove() completes.
So there is no reason to do this explicitly in the driver itself.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20230319100620.295849-2-u.kleine-koenig@pengutronix.de


Signed-off-by: Rob Herring <robh@kernel.org>