At this time Protected attributes cannot be changed to become non-protected attributes, nor can they be deleted. You could instead deactivate them then either rename and/or adjust their sort order such that they drop to the end of your list of custom ticket attributes.
You could also submit an enhancement request, if the above recommendations are not preferred, to ask for the ability to delete PCAs. Please note that TeamDynamix is not able to guarantee that any submitted request for sure will be implemented, however you will be contacted should your request be selected for inclusion in a future version of the product.
Please let me know if you have any additional questions or concerns about this or anything I've mentioned above.