Bluetooth: btuart-cs: Remove empty destruct cb

The destruct callback is optional and we provide an empty callback so
remove it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>