delete unnecessary using directive

Author: smdn

src/Smdn.Net.AddressResolution/Smdn.Net.AddressResolution/MacAddressResolver.NetworkScanning.cs

@@ -1,6 +1,5 @@
 // SPDX-FileCopyrightText: 2022 smdn <smdn@smdn.jp>
 // SPDX-License-Identifier: MIT
-using System;
 using System.Diagnostics;
 using System.Net;
 using System.Net.NetworkInformation;