Bluetooth: Mesh: Silence proxy not connected log

When performing node reset over a proxy connection, the disconnection is
asynchronous, and will finish after the reset callback goes to the
application. If the application restarts provisioning in this callback,
the disconnected-event is triggered after PB GATT is reactivated, and
this error is printed without any faults actually occurring.

Signed-off-by: Trond Einar Snekvik <Trond.Einar.Snekvik@nordicsemi.no>