Trying to removing message icons fails

[Sesquipedalian]

If the option to customize message icons is enabled, one can add new icons and modify existing icons, but trying to remove them doesn't work.

To reproduce this bug:

1. Go to ...index.php?action=admin;area=smileys;sa=settings and enable the "Enable customized message icons" setting.
2. Go to ...index.php?action=admin;area=smileys;sa=editicons.
3. Try adding a new icon. See that it works.
4. Try modifying any of the icons in the list. See that it works.
5. Click the check box next to any of the icons in the list, and then click the "Remove icon" button. See how when the page reloads, the icon has not be removed.