lib: remove enabled flag for bfd sessions

lib: remove enabled flag for bfd sessions

Currently this flag is only helpful in an extremely rare situation when
the BFD session registration was unsuccessful and after that zebra is
restarted. Let's remove this flag to simplify the API. If we ever want
to solve the problem of unsuccessful registration/deregistration, this
can be done using internal flags, without API modification.

Also add the error log to help user understand why the BFD session is
not working.

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