Remove TODO that is not gonna be done

Previously the idea was to cache sanitize results externally (think,
in Fontconfig) and avoid resanitizing every time.  That's, not a good
idea.

src/hb-open-type-private.hh

@@ -283,8 +283,6 @@ struct hb_sanitize_context_t :
   {
     bool sane;
 
-    /* TODO is_sane() stuff */
-
     init (blob);
 
   retry: