Node group useful_demand_electric and related Sankey queries should be removed

[kndehaan]

The node group useful_demand_electric is used in some useful demand nodes, subsequently queried in old Sankey queries in folder sankey_primary,_final,_useful_demand.

These queries are not used anymore, therefore this node group is also not used anymore. Proposed is to remove:

• Node group useful_demand_electric
• Old queries that use this node group

[linear]

MD-701