shared/gatt-client: Implement bt_gatt_client_unregister_notify.
shared/gatt-client: Implement bt_gatt_client_unregister_notify.

This patch implements bt_gatt_client_unregister_notify, which is used to remove
a previous registered notification/indication handler. This function decreases
the per-characteristic count of handlers, and if the count drops to 0, a write
request is sent to the CCC descriptor to disable notifications/indications.

mapfau/bluez,silent-snowman/bluez,ComputeCycles/bluez,silent-snowman/bluez,ComputeCycles/bluez,mapfau/bluez,pkarasev3/bluez,pkarasev3/bluez,pstglia/external-bluetooth-bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,mapfau/bluez