drop unnecessary SYN backlog configuration

This doesn't do anything on modern Linux kernel versions when SYN
cookies are enabled.

sysctl.d/local.conf

@@ -15,13 +15,14 @@ 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
+net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 31
 
 # 15s with initial 1s RTO
 net.ipv4.tcp_synack_retries = 3
+net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 15
 
 # 102.2s with minimum 0.2s RTO
 net.ipv4.tcp_retries2 = 8