As it have been said, you must eliminate the entries of that unit in:
export_descr_unit
export_descr_buildings

Problems with CTD in campaign can come from the presence of that eliminated unit in:
descr_strat
descr_rebel_factions (if it is also assigned to slave faction)
descr_mercenaries (if it is a mercenary unit)

Why do you want to eliminate that unit?
The answer would help to guide you. Perhaps it is not necessary to eliminate completely the unit. Examples:
- You don't want it in custom battles: put no_custom as an attribute of the unit in export_descr_unit (it won't be usable in custom battles)
- You don't want it in campaigns: simply, delete the unit in export_descr_buildings (it won't be recruitable)

Good luck!