I accidentally added it and the delete button doesn't appear in the action column to remove it.
I've seen this problem before. Try doing a hard cache clear in your browser or switching to a different browser.
If writing any kind of script does not appeal to you, another option is to install the Apache Chemistry Workbench. You can connect to the repository and adjust the ACL using that tool without code.
Or maybe someone else will suggest how to get the button back. But at least you've got some options.
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