We refer to this as a risk outcome. Risk outcomes are related to risk categories; normally the relationship is 1:1, though it is possible to add more than one outcome to for each risk category. Regardless, to remove a risk outcome from a risk assignment, in the Risk Register double-click on the risk in which you want to remove a risk outcome assignment and click the Assign to tasks or resources tab(or Probabilities and Outcomes for qualitative risk assignment). Right-click on the risk outcome that you want to delete and from the shortcut menu click Delete row.
You will have to repeat this for each task or resource to which the risk is assigned.