Agenda: Simplify old fix of face for deadline warning 0 day

* lisp/org-agenda.el (org-agenda-get-deadlines): Fix dfrac for the
case of wdays being 0.  Don't pass wdays to org-agenda-deadline-face,
like before the old fix.
(org-agenda-deadline-face): Revert to old state that was without
wdays.

This allows a deadline warning period of "-0d" to work also with a
custom configuration like:
(setq org-agenda-deadline-faces
      '((1.0001 . org-warning)              ; due yesterday or before
        (0.0    . org-upcoming-deadline)))  ; due today or later
For org-agenda-deadline-faces left at default and all other settings
not exceeding 1.0, the face for a deadline warning of any length
remains untouched by this patch.

references to history:
* commit of the old fix
  - http://orgmode.org/w/?p=org-mode.git;a=commitdiff;h=d0d6325
  - git show d0d6325
* mailing list old thread
  - http://thread.gmane.org/gmane.emacs.orgmode/5753
  - http://lists.gnu.org/archive/html/emacs-orgmode/2008-02/msg00395.html