Remove a duplicate (second) manager from Employment Assignment

Summary:

Associate was given a second manager with an 'invalid' manager type in 2017, then the 'invalid' type was inactivated. We need to remove the second manager line on the record but can't.

Content (required):

We have an associate who was transferred in 2017 and was assigned to the same manager twice (same name but one supervisor type was 'line manager' and one was 'invalid manager') on the same assignment record. Then in 2019 the manager type 'invalid manager' was delimited without that old record being removed (it was the only one). Now we are having issues deleting that second supervisor line on the associate's record. I activated the supervisor type, but the validation rule says that there can only be one manager on a record; if we try to just remove