Revert "networkd: change UseMTU default to true. (#6837)" (#6950)

This reverts commit 22043e4317.
UseMTU is broken on real hardware and should not be enabled by default.

src/network/networkd-network.c

@@ -223,7 +223,8 @@ static int network_load_one(Manager *manager, const char *filename) {
          * even if they are commented in the man?
          * These vars might be overwriten by network_apply_anonymize_if_set */
         network->dhcp_vendor_class_identifier = false;
-        network->dhcp_use_mtu = true;
+        /* NOTE: from man: UseMTU=... Defaults to false*/
+        network->dhcp_use_mtu = false;
         /* NOTE: from man: UseTimezone=... Defaults to "no".*/
         network->dhcp_use_timezone = false;