Remove variable only used for an assert

When compiled with -DNDEBUG, we get warnings about set-but-unused variables.
In general, it's not something we care about, but since removing those
variables arguably makes the code nicer, let's just to it in this case.

src/network/netdev/fou-tunnel.c

@@ -64,14 +64,10 @@ static int netdev_fill_fou_tunnel_message(NetDev *netdev, sd_netlink_message **r
 static int netdev_fou_tunnel_create(NetDev *netdev) {
         _cleanup_(sd_netlink_message_unrefp) sd_netlink_message *m = NULL;
         uint32_t serial;
-        FouTunnel *t;
         int r;
 
         assert(netdev);
-
-        t = FOU(netdev);
-
-        assert(t);
+        assert(FOU(netdev));
 
         r = netdev_fill_fou_tunnel_message(netdev, &m);
         if (r < 0)

src/network/netdev/wireguard.c

@@ -430,11 +430,8 @@ static void resolve_endpoints(NetDev *netdev) {
 }
 
 static int netdev_wireguard_post_create(NetDev *netdev, Link *link, sd_netlink_message *m) {
-        Wireguard *w;
-
         assert(netdev);
-        w = WIREGUARD(netdev);
-        assert(w);
+        assert(WIREGUARD(netdev));
 
         (void) wireguard_set_interface(netdev);
         resolve_endpoints(netdev);