There's some stuff about removing units here:
http://www.twcenter.net/wiki/Removing_Units

basically if you remove model from descr_model_battle.txt you have to remove/change every unit that uses that model as a soldier type in export_descr_unit.txt

If you remove a unit from export_descr_unit.txt you have to remove all references to that unit in
export_descr_buildings.txt
descr_strat.txt
descr_rebel_factions.txt
descr_mercenaries.txt
and possibly in script if you are working on a mod that has one.

Most errors should give you a message if you are using -show_err on your shortcut.