No longer advertise or negotiate any consensus method before 13.

Implements proposal 215; closes ticket 10163.

Why?  From proposal 215:

   Consensus method 1 is no longer viable for the Tor network.  It
   doesn't result in a microdescriptor consensus, and omits other
   fields that clients need in order to work well.  Consensus methods
   under 12 have security issues, since they let a single authority
   set a consensus parameter.
...
   For example, while Tor 0.2.4.x is under development, authorities
   should really not be running anything before Tor 0.2.3.x.  Tor
   0.2.3.x has supported consensus method 13 since 0.2.3.21-rc, so
   it's okay for 0.2.4.x to require 13 as the minimum method.  We even
   might go back to method 12, since the worst outcome of not using 13
   would be some warnings in client logs.  Consensus method 12 was a
   security improvement, so we don't want to roll back before that.
Remove support for generating consensuses with methods <= 9.

The last patch disabled these; this one removes the code to implement
them.
Remove implementation code for all pre-13 consensus methods.

Also remove a test for the way that we generated parameter votes
before consensus method 12.
changes file for bug 10163 / proposal 215.
Remove the assigned-but-unused chosen_named_idx local variable

It had been used in consensus method 1.  But now that 13 is the
minimum (see #10163), we don't need it around.

Found by sysrqb.
Merge remote-tracking branch 'public/bug10163'