Clean up initialization and customization of horizontal scroll bars.

Clean up initialization and customization of horizontal scroll bars.

* frame.el (frame-initialize): Remove horizontal-scroll-bars
from frame-initial-frame-alist.
* scroll-bar.el (previous-horizontal-scroll-bar-mode)
(horizontal-scroll-bar-mode-explicit)
(set-horizontal-scroll-bar-mode, get-horizontal-scroll-bar-mode)
(toggle-horizontal-scroll-bar): Remove.
(horizontal-scroll-bar-mode): Remove defcustom.
(horizontal-scroll-bar-mode): Fix doc-string.
(scroll-bar-toolkit-scroll)
(scroll-bar-toolkit-horizontal-scroll): Add doc-strings stubs.
* buffer.c (scroll-bar-height): Fix typo in doc-string.
* frame.c (Vdefault_frame_horizontal_scroll_bars): Remove
variable.
* nsfns.m (Fx_create_frame):
* w32fns.c (Fx_create_frame):
* xfns.c (Fx_create_frame): Default horizontal scroll bars to
nil.