Remove unused popup manager implementation

I haven't investigated, but I imagine that, long ago, we had a little status icon in the old status bar that web browsers used to have at the bottom of the window. It probably indicated the number of popups blocked on the current page and functioned to show or hide the popups. But it doesn't exist anymore. This code hasn't done anything for a long time. The show function is unimplemented, which indicates it probably never survived the port to WebKit2. Bye.