net: remove CService::ToString() use ToStringAddrPort() instead

Both methods do the same thing, so simplify to having just one.

`ToString()` is too generic in this case and it is unclear what it does,
given that there are similar methods:
`ToStringAddr()` (inherited from `CNetAddr`),
`ToStringPort()` and
`ToStringAddrPort()`.

src/test/fuzz/netaddress.cpp

@@ -79,7 +79,6 @@ FUZZ_TARGET(netaddress)
     const CService service{net_addr, fuzzed_data_provider.ConsumeIntegral<uint16_t>()};
     (void)service.GetKey();
     (void)service.GetPort();
-    (void)service.ToString();
     (void)service.ToStringAddrPort();
     (void)service.ToStringPort();
     (void)CServiceHash()(service);