Remove broken logic in backwards walks in validate_visible_area()

Remove broken logic in backwards walks in validate_visible_area()

In validate_visible_area() it was assumed that gtk_tree_path_prev()
would always return the correct path of the preceding node.  This is
obviously not true.  The if-clause has been removed so that we now
always use _gtk_tree_view_find_path() to get the path from the tree,
node.