Make GimpDockColumns listen to "dock-removed", not "dockbook-removed",
when trying to figure out when to destroy itself. Fixes some crashes
when rearranging the UI, for example when doing this step-by-step:

1. Have two dock windows with one dockable each, say A and B
2. Move A to B's dock window and make it multi-column
3. Try to detach B, will result in a crash