Remove CentOS 6 support for Tor Browser 10.5

#40080 (closed) was a bit unclear when saying "ESR 78 is the last Tor Browser series to support CentOS 6." What it meant was that Tor Browser 10.0 is this last series (which is why we removed the CentOS 6 compat patch while rebasing to Firefox 80.0b2)

This ticket is for removing CentOS 6 support for Tor Browser 10.5