Bugfix: Don’t run into an endless loop when killing con with children (Thanks mseed)

Bugfix: Don’t run into an endless loop when killing con with children (Thanks mseed)

When a tabbed container had more than one child and at least the first one
supported WM_DELETE, i3 entered an endless loop when killing that tabbed
container. This was due to tree_close only sending WM_DELETE without actually
removing the child, while the loop in tree_close assumed that with every call
of tree_close one child would be removed.