Avoid obsolete special default variables like default-major-mode.

Avoid obsolete special default variables like default-major-mode.
* display.texi (Line Height):
* emacs-lisp-intro.texi (Text and Auto-fill, Mode Line):
* display.texi (Useless Whitespace, Text Display):
* custom.texi (Init Examples): Avoid obsolete special default variables
like default-major-mode.