Deleting a Definition 
The delete option is only available for those definitions that you create in the Rules Manager.
Note
Once you upload the changes made to the DC, the definition can not be deleted in the rules manager.
If definition is not being used only then, the definition gets deleted.
End of the note.
Procedure
In the Rules Navigator view, select the project node and expand the ruleset or flow ruleset node.
Choose Definitions.
The definition opens in the Editor Window.
Select the radio button next to the definition.
Choose the Delete button.
In the dialog box that appears, choose Ok.
The selected definition gets deleted. If the definition is being used, then all usage references get displayed.