tcp: remove unused TCP_SYNQ_INTERVAL definition

Currently TCP_SYNQ_INTERVAL is defined but never used.

According to "git log -S TCP_SYNQ_INTERVAL net-next/main" it seems
the last references to TCP_SYNQ_INTERVAL were removed by 2015
commit fa76ce73 ("inet: get rid of central tcp/dccp listener timer")

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>