Hello Michael,
You can not delete a custom field you can only hide them.
Delete Fields
You cannot delete a field. Instead, edit a field while viewing the Fields summary from your workspace, and select No in the Visible (All Projects) drop-down.

Alternatively, if you have the need for another custom field of the same type, you may edit the field and change the name to repurpose it.
Please let us know if you have any further questions.
Thank you,
Christian