To remove custom fields please follow these steps.
Go to Project settings by clicking on settings icon on top right corner
Click on custom fields from left menu
Select user story from first drop down "Artifact Type"
At bottom of page you will see existing custom fields.
If you mouse over the custom field "Acceptance criteria", you will see at end of row a DELETE icon, pressing this icon will remove this custom field from your project.
See attached screen shot for reference.