revert(os_fingerprint): undo ip_no_pmtu_disc=1 for windows — was incorrect

ip_no_pmtu_disc controls PMTU discovery for UDP/ICMP paths only.
TI=Z originates from ip_select_ident() in the kernel TCP stack setting
IP ID=0 for DF=1 TCP packets — a namespace-scoped sysctl cannot change this.
The previous commit was based on incorrect root-cause analysis.

decnet/os_fingerprint.py

@@ -48,7 +48,7 @@ OS_SYSCTLS: dict[str, dict[str, str]] = {
         "net.ipv4.tcp_window_scaling": "1",
         "net.ipv4.tcp_sack": "1",
         "net.ipv4.tcp_ecn": "0",
-        "net.ipv4.ip_no_pmtu_disc": "1",   # avoid TI=Z: forces non-zero IP IDs
+        "net.ipv4.ip_no_pmtu_disc": "0",
         "net.ipv4.tcp_fin_timeout": "30",
     },
     "bsd": {