Remove one library picture.

The brand slots are refused: logo and logo_dark are slots, not entries — a template binds to them by name, so the way to change one is to upload over it, and "delete" would leave every starter's mark empty with no way back through this API.

Templates already bound to the picture are left alone on purpose. The binding resolves to nothing, the layer hides with an image_missing warning, and the preview says so — which is visible and fixable, unlike a scan that rewrote documents on the author's behalf.