Can add project->package link but not remove it

[Matthew Paul Thomas]

1. Go to <https://launchpad.net/server-papercuts/trunk/+ubuntupkg>.
2. Try to remove the link to the "papercut" package, which is not a package of that project.

What happens:
1. There is a "Source Package Name:" field containing "papercut", and no obvious way to remove it.
2. Blanking the field and choosing "Update" returns an error message, "Required input is missing".

What should happen:
1. there is an obvious control (like a "Remove" button) for removing the package link; and/or
2. blanking the field and choosing "Update" removes the package link.