[Feature]: remove vZERO cruft

[cfe316]

Feature description

vZERO, a JuMP variable which is always zero, was necessary in a few places in early versions of GenX/JuMP. It is not necessary any longer. As cruft, it should be removed.

Motivation

Removing useless code makes the rest more readable.

Implementation strategy

No response

Additional context

No response

[lbonaldo]

Resolved by #742 on develop.