lib: remove confusing bfd TTL API

lib: remove confusing bfd TTL API

There are two APIs to control the expected number of hops for a BFD
session – `bfd_sess_set_mininum_ttl` and `bfd_sess_set_hop_count`.
The former is very confusing, as it takes an expected TTL in the
BFD packet which is actually a protocol internal value. The latter is
simple and straightforward – it takes an expected number of hops, which
is always 1 for single-hop and >1 for multi-hop.

As the former API is not used anywhere, just remove it to avoid any
confusion.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>