platform/x86: acer-wmi: Cleanup accelerometer device handling

Cleanup accelerometer device handling:
-Drop acer_wmi_accel_destroy instead directly call input_unregister_device.
-The information tracked by the CAP_ACCEL flag mirrors acer_wmi_accel_dev
 being NULL. Drop the CAP flag, this is a preparation change for allowing
 users to override the capability flags. Dropping the flag stops users
 from causing a NULL pointer dereference by forcing the capability.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20201019185628.264473-3-hdegoede@redhat.com