Hi Sam,
Thanks for reaching out to us. With regards to your concern about remove a url_custom_parameter, you can use AdGroupService with Mutate operation.
As for the REMOVE operation concern, you can call REMOVE on the custom parameters. If you perform remove() operations, it will remove the whole ad group. You can only remove settings by updating the ad group please refer to this code example.
If you can use update_mask on custom_parameters and set the custom_parameters to empty, that would work too. You can find more information on Field Masks here and and code sample to update adgroup.
Let me know if you have any questions.
Thanks,
Nirmita Patel Google Ads Team