babeld: remove "parasitic" mode.

babeld: remove "parasitic" mode.

This is the functionality described in Appendix C of RFC 6126.  Its
main purpose is to avoid keeping a full source table, which makes it
possible to implement a subset of Babel in just a few hundred lines of
code.  However, in Quagga the code for maintaining the source table is
already there, and a parasitic implementation can be simulated using
filtering -- so it makes little sense to keep the functionality.