wallet: do not log by default if we're not asked to log to console

This means monero-wallet-rpc still does, but the user level program
does not.

src/wallet/wallet_args.cpp

@@ -179,6 +179,10 @@ namespace wallet_args
     {
       mlog_set_log(command_line::get_arg(vm, arg_log_level).c_str());
     }
+    else if (!log_to_console)
+    {
+      mlog_set_categories("");
+    }
 
     if (notice)
       Print(print) << notice << ENDL;