Exclude psu_id from the -4orcing process.  #4571.

psu_id is set further up the pipeline (specifically, as an
ApplyGlobalValuesFilter[1] in ncs_mdes_warehouse).  Setting it here
interferes with that process.

[1]: https://github.com/NUBIC/ncs_mdes_warehouse/blob/master/lib/ncs_navigator/warehouse/filters/apply_global_values_filter.rb#L45-L61