bgpd: remove cumulus specific code

bgpd: remove cumulus specific code

The BFD function `bgp_bfd_is_peer_multihop` will no longer exist and now
both code paths are equal.

Longer explanation:
Cumulus was previously using the BFD function to help determine whether a
peer is multi hop or not, because there is a configuration to set BFD
to use single or multi hop.

Current BFD code can automatically pick between single/multi hop by
using the protocol information and so it is a good idea to have that
tested/used than relying on yet another duplicated information.

(BFD extracts the TTL information from protocol and selects
single/multi hop based on that)

Signed-off-by: Rafael Zalamena <rzalamena@opensourcerouting.org>