tcp_bbr: remove bbr->tso_segs_goal

Its value is computed then immediately used,
there is no need to store it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>