Replace (default-value 'enable-multibyte-characters) with t

Replace (default-value 'enable-multibyte-characters) with t

Since Emacs-24, (default-value 'enable-multibyte-characters) can't be
changed any more, so we can remove all code which checks this value.

* lisp/x-dnd.el (x-dnd-handle-file-name):
* lisp/w32-fns.el (w32-set-default-process-coding-system):
* lisp/textmodes/ispell.el (ispell-send-string):
* lisp/term/internal.el (dos-codepage-setup):
* lisp/tar-mode.el (tar-header-block-tokenize, tar--extract):
* lisp/startup.el (command-line):
* lisp/server.el (server-process-filter):
* lisp/net/browse-url.el (browse-url-file-url):
* lisp/menu-bar.el (menu-bar-options-menu, menu-bar-describe-menu):
* lisp/mail/sendmail.el (mail-setup):
* lisp/mail/rmail.el (rmail-mode, rmail-get-new-mail):
* lisp/mail/rfc2047.el (rfc2047-encode-message-header):
* lisp/mail/emacsbug.el (report-emacs-bug):
* lisp/international/mule-diag.el (mule-diag):
* lisp/files.el (find-file-noselect-1):
* lisp/dos-fns.el (dos-set-default-process-coding-system):
* lisp/arc-mode.el (archive-mode):
* lisp/international/mule-cmds.el (mule-menu-keymap)
(set-default-coding-systems, set-language-info)
(set-language-environment, standard-display-european-internal)
(set-locale-environment): Simplify code.