I think it's quite hard for users to find that useful (but hidden) feature by themselves intuitively because the average user would not expect the Submit button to be required *in conjunction* with the Ctrl+Del shortcut.
In an ideal world, I think the Ctrl+Del shortcut would be sufficient to remove an entry from the dropdown menu but I understand there may be an underlying limitation (e.g. in the code) which forces this behaviour.
I'm happy with the way that it is, now that I know the secret, but I suspect that some other users may try a few different obvious methods to clear up their dropdown menu and then simply give up, not realizing that they need to hit Submit to apply the changes! :)