Bug 679616 - Stop whitelisting dead mozilla.widget.render-mode pref in about:support. r=bas

toolkit/content/aboutSupport.js

@@ -66,7 +66,6 @@ const PREFS_WHITELIST = [
   "general.useragent.",
   "gfx.",
   "html5.",
-  "mozilla.widget.render-mode",
   "layers.",
   "javascript.",
   "keyword.",