Removed unused using.

Author: dlemstra

tests/Magick.NET.Tests/ResourceLimitsTests/TheLimitMemoryMethod.cs

@@ -1,7 +1,6 @@
 // Copyright Dirk Lemstra https://github.com/dlemstra/Magick.NET.
 // Licensed under the Apache License, Version 2.0.
 
-using System;
 using ImageMagick;
 using Xunit;