argument not used in _load_constraints(

In function:
```
_load_constraints(model::GeometricConicForm, src, indexmap, cone_offset, i, cache)
```

`i` is not used at all. We could remove it but I suspect this should be used and has been forgotten.

Start by locating the `_load_constraints(model::GeometricConicForm, src, indexmap, cone_offset, i, cache)` definition and its callers. Trace how `i` relates to the constraint-loading flow, then determine whether the argument should affect behavior or be removed; done means the signature and behavior are consistent.

Vom Indexierungsmodell aus dem Issue-Text verfasst.