Remove SupportsSIUnitSystem => true property overrides

[lipchev]

fixes#1463

[lipchev]

Duplicate of #1534 which got deleted when release/v6 went away..

As expected, there are only the

ElectricApparentEnergy
ElectricReactiveEnergy
FuelEfficiency

which override this property (and also [Skip] the corresponding tests). The only other tests that are skipped are for the BaseUnit_HasSIBase (as noted in the linked issue).