net: remove CNetAddr::ToString() and use ToStringAddr() instead

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

Further, `CService` inherits `CNetAddr` and `CService::ToString()`
overrides `CNetAddr::ToString()` but the latter is not virtual which
may be confusing. Avoid such a confusion by not having non-virtual
methods with the same names in inheritance.

src/test/fuzz/netaddress.cpp

@@ -70,7 +70,6 @@ FUZZ_TARGET(netaddress)
         assert(net_addr.GetNetwork() == Network::NET_ONION);
     }
     (void)net_addr.IsValid();
-    (void)net_addr.ToString();
     (void)net_addr.ToStringAddr();
 
     const CSubNet sub_net{net_addr, fuzzed_data_provider.ConsumeIntegral<uint8_t>()};