Remove pathbias code

This is blocked by legacy/trac#26886 (moved).

mikeperry in comment:7:issue:27228:

I think it is fine to remove all of the pathbias code once we get a tagging resistant cipher in place, such as https://gitweb.torproject.org/torspec.git/tree/proposals/295-relay-crypto-with-atl.txt Hoping for further guidance in any case i will just try this. We'll see if i end up with a stable tor.

Once this is merged unresolved pathbias bugs become obsolete and it might make legacy/trac#16764 (moved), legacy/trac#21084 (moved), legacy/trac#25783 (moved) easier.