fuzz: reduce possible networks check

If an address classifies as `IsRFC4193()`, then it cannot be
`NET_ONION` (Tor v3), thus remove that condition from the assert.

src/test/fuzz/netaddress.cpp

@@ -54,7 +54,7 @@ FUZZ_TARGET(netaddress)
     (void)net_addr.IsRFC3927();
     (void)net_addr.IsRFC3964();
     if (net_addr.IsRFC4193()) {
-        assert(net_addr.GetNetwork() == Network::NET_ONION || net_addr.GetNetwork() == Network::NET_INTERNAL || net_addr.GetNetwork() == Network::NET_UNROUTABLE);
+        assert(net_addr.GetNetwork() == Network::NET_INTERNAL || net_addr.GetNetwork() == Network::NET_UNROUTABLE);
     }
     (void)net_addr.IsRFC4380();
     (void)net_addr.IsRFC4843();