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>