display-page-tree: remove page on remove action

Instead of calling rb_display_page_delete_thyself, call
rb_display_page_remove. This allows the page to perform any required
logic before it deletes itself. Audioscrobbler radio sources will now
remain removed after application restart, as they are now able to save
the updated station list upon removal.

https://bugzilla.gnome.org/show_bug.cgi?id=707525