Remove AAAA record for check.torproject.org

It looks like check.torproject.org's IPv6 address record is causing problems for users: https://www.fredericjacobs.com/blog/2016/08/17/torcheck-ipv6/

I think he's right that until we have someone who can update check.torproject.org, we should remove the IPv6 entry for it, so that IPv6 exit traffic doesn't hit check.torproject.org.