Remove /tor/dbg-stability.txt URL served by directory authorities
Tor 0.2.1.6-alpha added a /tor/dbg-stability.txt URL on directory authorities to help debug WFU and MTBF calculations. But nobody is using it, and directory authorities shouldn't expose any more data than necessary. Also, the better approach to debugging how relays are included in votes is proposal 164. We should remove this URL.