Remove our content policy hack for #8725
Back then Yawning implemented a hack to defend leaking installed Tor Browser resources to content (see: #8725 (moved)). Thus got upstreamed during the ESR60 timeframe (https://bugzilla.mozilla.org/show_bug.cgi?id=863246) and is available in Tor Browser now. We can think about removing this workaround from Torbutton then.