Remove extra unused heat node

Removes "energy heat remainder from heat network unused steam hot
water" in favour of connecting extra agriculture and industry heat
directly to the "energy heat network steam hot water" node.
Removed remaining 'unused heat' nodes
Removed useful_demand and useful_demand_heat_cold from the groups for energy_heat_network_unused_steam_hot_water. The fact that this overflow was part of these groups made no sense. For the primary demand calculation, it is needed that the node stays in the final_demand_group because the primary demand of the CHPs in agriculture and industry is only due to the steam_hot_water that is actually used in the sectors.
Merge pull request #877 from quintel/unused-heat

Unused heat