Exclude psu_id from the -4orcing process (#4571)

`psu_id` is set further up the pipeline (specifically, as an [ApplyGlobalValuesFilter](https://github.com/NUBIC/ncs_mdes_warehouse/blob/master/lib/ncs_navigator/warehouse/filters/apply_global_values_filter.rb#L45-L61) in ncs_mdes_warehouse).  Setting it here interferes with that process.