Bluetooth: GATT: Unregister CCC when unregistering services

Fix issue with bt_gatt_service_unregister not clearing CCC information.

If the service is unregistered while an unbonded peer is connected and
subscribed then this CCC would be restored again when registering the
service even if the peer is now disconnected.

If the service is unregistered with bonded peers CCCs stored in settings
then this value would never be cleared. If the service is registered
again then the peer would receive a service changed for this service,
but it's CCC value would still be enabled.

Fixes: #26924

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>