bgpd: Remove AS Path limit/TTL functionality

* draft-ietf-idr-as-pathlimit doesn't seem to have gone anywhere, and its
  author does not think it will make progress in IDR. Remove all support
  introduced for it, but leave stubs for the commands to avoid breaking
  any configurations.

  Basically reverts cecab5e9725792e60a5e4b473e238a14cd85815d.
7 files changed