The NMDevice dispose() function contained some badly-duplicated logic
about when to deactivate a device on its last ref.  This logic should
only run when the device is removed by the manager, since the  manager
controls the device's life-cycle, and the manager knows best when to
clean up the device.  But since it was tied to the device's refcount,
it could have run later than the manager wanted, or not at all.

It gets better.  Dispose duplicated logic that was already done in
nm_device_cleanup(), and then *called* nm_device_cleanup() if the
device was still activated and managed.  But the manager already
unmanages the device when removing it, which triggers a call to
nm_device_cleanup(), takes the device down, and resets the IPv6
sysctl properties, which dispose() duplicated too.  So by the time
dispose() runs, the device should already be unmanaged if the
manager wants to deconfigure it, and most of the dispose() code
should be a no-op.

Clean all that up and remove duplicated functions.  Now, the flow
should be like this:

1) manager decides to remove the device and calls remove_device()
2) if the device should be deconfigured, the manager unmanages
   the device
3) the NMDevice state change handler tears down the active connection
   via nm_device_cleanup() and resets IPv6 sysctl properties
4) when the device's last reference is finally released, only internal
   data members are freed in dispose() because the device should
   already have been cleaned up by the manager and be unmanaged
5) if the device should be left running because it has an assumable
   connection, then the device is not unmanaged, and no cleanup
   happens in the state change handler or in dispose()