Delete duplicated code

This piece of code below appears twice in quack.el.

(defconst quack-pltish-paren-face 'quack-pltish-paren-face)
(defface  quack-pltish-paren-face
  '((((class color) (background light)) (:foreground "red3"))
    (((class color) (background dark))  (:foreground "red1"))
    (((class grayscale))                (:foreground "gray"))
    (t                                  ()))
  "Face used for parentheses when `quack-fontify-style' is `plt'."
  :group 'quack)