It would be better if the user could choose whether he/she wants to install the Single Approver definition or not, since currently it is very inconvenient to delete it if it's unnecessary.
If you delete it from UI then after the application server restart the Single Approver definition appears again.
With the following workarounds the definition can be deleted:
1) Remove single-approver-definition.xml from kaleo-web plugin
2) Delete the reference from the kaleo-spring.xml in kaleo-web plugin