Delete window-combination-p; tweaks to window-top-child and window-left-child.

Delete window-combination-p; tweaks to window-top-child and window-left-child.

* lisp/window.el (window-combination-p): Function deleted; its
side-effect is not used in any existing code.
(window-combinations, window-combined-p): Call window-*-child
directly.

* window.c (Fwindow_live_p, Fwindow_frame, Fframe_root_window)
(Fminibuffer_window, Fwindow_buffer, Fwindow_splits)
(Fset_window_splits, Fwindow_nest, Fset_window_nest)
(Fwindow_use_time, Fwindow_total_size, Fwindow_normal_size)
(Fwindow_new_normal, Fwindow_left_column, Fwindow_top_line)
(Fwindow_margins, Fwindow_fringes, Fwindow_scroll_bars)
(Fwindow_vscroll): Doc fix.
(Fwindow_top_child, Fwindow_left_child): Eliminate a nil default
argument, since it makes no sense to pass a live window and for
consistency with window-child.