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>

include/net/tcp.h

@@ -161,8 +161,6 @@ void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define MAX_TCP_KEEPCNT		127
 #define MAX_TCP_SYNCNT		127
 
-#define TCP_SYNQ_INTERVAL	(HZ/5)	/* Period of SYNACK timer */
-
 #define TCP_PAWS_24DAYS	(60 * 60 * 24 * 24)
 #define TCP_PAWS_MSL	60		/* Per-host timestamps are invalidated
 					 * after this time. It should be equal