symptom:
(custom-face-attributes-get 'font-lock-keyword-face nil)
 ==> (:foreground "Cyan1")

(set-face-attribute 'font-lock-keyword-face nil
            :family "verdana")

(custom-face-attributes-get 'font-lock-keyword-face nil)
 ==> (:foreground "Cyan1" :family "verdana")

--------------------------------------------------------------

(set-face-attribute 'font-lock-keyword-face nil
            :family 'unspecified)

(custom-face-attributes-get 'font-lock-keyword-face nil)
 ==> (:foreground "Cyan1" :family "verdana")             ;; wasn't removed

--------------------------------------------------------------


my solution:
modify faces.el file
===============================================================
738c738
<       (when (stringp family)
---
>       (when (or (stringp family)(eq family 'unspecified))
741c741
<       (when (stringp foundry)
---
>       (when (or (stringp foundry)(eq foundry 'unspecified))
===============================================================