net: remove spurious declaration of tcp_default_init_rwnd()

it doesn't actually exist...

Test: builds and 'git grep tcp_default_init_rwnd' comes up empty
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1376,7 +1376,6 @@ static inline void tcp_sack_reset(struct tcp_options_received *rx_opt)
 	rx_opt->num_sacks = 0;
 }
 
-u32 tcp_default_init_rwnd(u32 mss);
 void tcp_cwnd_restart(struct sock *sk, s32 delta);
 
 static inline void tcp_slow_start_after_idle_check(struct sock *sk)