You can only delete an apportionment if it is not being used in any rules.
If an apportionment is not being used in any rules, when you hover over it, you will see an edit or delete icon:
and you can simply click the trash icon to delete the apportionment.
If the apportionment is being used in a rule then you will only see an edit icon:
and you will not be able to delete the apportionment.