ACPI: bus: Drop redundant check in acpi_device_remove()

A bus remove callback is only ever called by the device core with a
bound driver. So there is no need to check if the driver is non-NULL.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
[ rjw: Added empty code line after if () ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>