ACPI: APEI: Drop unsetting driver data on remove

Since commit 0998d063

 ("device-core: Ensure drvdata = NULL when no
driver is bound") the driver core cares for cleaning driver data, so
don't do it in the driver, too.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>