Cannot remove signup-enabled products from cart, if event is closed and user doesn't have "purchase signups for other users"

Issue Summary

When a user has a signup-enabled product from cart and the event is sold out, there is no way for the user to remove it from their cart unless they have the "purchase signups for other users" permission.

A related bug (or quirk, I suppose) is that the only way to remove a sold-out event from your cart is to set the quantity to zero -- trying to use the "Remove" button returns "We're sorry, but there are 0 spots left for the [event type] [event name]."

But, since uc_signup disables the quantity field for users without the "purchase signups for other users" permission, this makes it impossible to remove the item from the cart.

A real-world scenario would be: the user adds an item to their cart, but doesn't complete the purchase. They come back a few months later to find the expired event still in their cart, but now they are not able to remove it.

This could probably be fixed by allowing the "Remove" button on the cart pane to remove closed events.