If it's an ABB supplied positioner the ext. axis is booted into the system with some parameters as "hidden" so deleting what you can will most likely cause a sysfail.
Easiest as Reboot mentions is to create a new system without the external axis (option) and reboot the robot with the new system. At that point you can remove the external axis without an resolver error, however, you will most likely get the PTC / overtemp error in which case you need to jumper the PTC signal that used to go to the external axis.