m_linkChangeCallbacks in CsmaNetDevice is not used properly

m_linkChangeCallbacks should be triggered when a device is attached and detached from a channel. CsmaNetDevice has an attach function CsmaNetDevice::Attach (Ptr<CsmaChannel> channel) used to attach the device to a channel. This function calls CsmaNetDevice::NotifyLinkUp () which triggers m_linkChangeCallbacks callback and also sets m_LinkUp to true. But there is no Detach () function in CsmaNetDevice. Currently, the device is detached from channel using CsmaChannel::Detach (). This function does not trigger m_linkChangeCallbacks.