GATT: Deregister notification when a GATT connection is closed

Precondition:
1. Pair with specific HOGP mouse.

Steps:
1. Pair with HOGP mouse (iBuffalo BSMBB09DS)
2. Unpair the mouse.
3. Pair the mouse again.

Failure:
Pairing is successful, but the mouse won't connect.

Root Cause:
Some HOGP mice send notifications before the stack writes the
client characteristic configuration descriptor.
The Bluetooth stack doesn't deregister these notifiactions
when disconnecting.

Fix:
Deregister notifications when the GATT connection closes.

Fixes: 29111689
Change-Id: I520faeaa5fc396381cb248d3a2c49efbffa12261