can: dev: can_restart(): remove unused code

Value assigned to variable err is overwritten at line
562: err = priv->do_set_mode(dev, CAN_MODE_START); before
it can be used.

Also, notice that this code has been there since 2014.

Addresses-Coverity-ID: 1227031
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>