remove unused SYN backlog configuration

This isn't used anymore despite inaccurate kernel configuration
documentation. The SYN_RECV queue is set based on the backlog value
just like the separate accept queue for established connections.

sysctl.d/local.conf

@@ -15,7 +15,6 @@ net.ipv4.tcp_notsent_lowat = 131072
 net.ipv4.tcp_fin_timeout = 30
 net.ipv4.tcp_rfc1337 = 1
 net.ipv4.tcp_tw_reuse = 1
-net.ipv4.tcp_max_syn_backlog = 65536
 
 # 31s with initial 1s RTO
 net.ipv4.tcp_syn_retries = 4