remove unused using

Look likes one of the updated packages were pulling an assembly with types declared
in NuGet.Frameworks assembly. Somehow this namespace ended up being imported
by accident in ConstTests.cs

Cecilifier.Core.Tests/Tests/Unit/ConstTests.cs

@@ -1,6 +1,5 @@
 using System.Text.RegularExpressions;
 using Mono.Cecil.Cil;
-using NuGet.Frameworks;
 using NUnit.Framework;
 
 namespace Cecilifier.Core.Tests.Tests.Unit