bgpd: get rid of afi_header_vty_out() & co.

bgpd: get rid of afi_header_vty_out() & co.

afi_header_vty_out() is easily replaced with vty_frame(), which means we
can drop a whole batch of "int *write" args as well as the entirety of
bgp_config_write_family_header().

=> AFI/SAFI config writing is now a lot simpler.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>