droptarget: Fix confusion around ::leave signal

The leave signal should not take any arguments,
don't pass the GdkDrop. Update the documentation,
fix the emission and update users to match.

This was showing up as crash when dragging files
over the places sidebar.

gtk/gtkdroptarget.c

@@ -471,7 +471,7 @@ gtk_drop_target_handle_crossing (GtkEventController    *controller,
           crossing->new_target == widget)
         return;
 
-      g_signal_emit (self, signals[LEAVE], 0, self->drop);
+      g_signal_emit (self, signals[LEAVE], 0);
       if (!self->dropping)
         gtk_drop_target_end_drop (self);
       gtk_widget_unset_state_flags (widget, GTK_STATE_FLAG_DROP_ACTIVE);
@@ -731,7 +731,6 @@ gtk_drop_target_class_init (GtkDropTargetClass *class)
   /**
    * GtkDropTarget::leave:
    * @self: the #GtkDropTarget
-   * @drop: the #GdkDrop
    *
    * The ::leave signal is emitted on the drop site when the pointer
    * leaves the widget. Its main purpose it to undo things done in

gtk/gtkplacessidebar.c

@@ -1912,7 +1912,6 @@ dnd_finished_cb (GdkDrag          *drag,
  */
 static void
 drag_leave_callback (GtkDropTarget *dest,
-                     GdkDrop       *drop,
                      gpointer       user_data)
 {
   GtkPlacesSidebar *sidebar = GTK_PLACES_SIDEBAR (user_data);