Remove redundant usings

Author: Saalvage

tests/dotnet/CSharp/CSharp.Tests.cs

@@ -3,10 +3,8 @@
 using System.Linq;
 using System.Reflection;
 using System.Runtime.InteropServices;
-using System.Text;
 using CSharp;
 using NUnit.Framework;
-using ValueType = CSharp.ValueType;
 
 [TestFixture]
 public unsafe class CSharpTests