spectrum: Remove Add[Spectrum]PropagationLossModel methods

Existing AddSpectrumPropagationLossModel modifies m_next field of the propagation loss model that is passed to it. In LTE helper the same m_fadingModel is passed twice to SetSpectrumPropagationLossModel: once for downlink channel, and once for uplink channel. As a result, if both downlink and uplink propagation loss models are SpectrumPropagationLossModel, m_fadingModel gets m_next pointing to the uplink propagation loss model. Because downlink channel already uses m_fadingModel, it ends up using uplink propagation loss model.

To avoid such errors in the future, Add[Spectrum]PropagationLossModel interfaces are removed. User has to link propagation loss models explicitly via SetNext and set them once via Set[Spectrum]PropagationLossModel.