Unused argument in `_add_pwl_constraint!`

[luke-kiernan]

In the ORDC market bid cost version of _add_pwl_constraint!, we're passing sos_status::SOSStatusVariable, but not actually using it anywhere in the function body. Code here. Either it's an oversight and we should remove the argument, or it's a bug.

[luke-kiernan]

This is fixed here in IOM/POM, but still present back in PSI. So this issue is in the wrong repo.