There are numerous IP's that are reported for github.com. The easiest way to remove all of the offending known_host entries appears to be:

  sed -i.github-removed '/AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa/d' ~/.ssh/known_hosts