Remove special case always adding 'using System;'

ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs

@@ -59,7 +59,7 @@ namespace ICSharpCode.Decompiler.CSharp
 				new DetectPinnedRegions(), // must run after inlining but before non-critical control flow transforms
 				new BlockILTransform {
 					PostOrderTransforms = {
-						new ExpressionTransforms() // for RemoveDeadVariableInit// for RemoveDeadVariableInit
+						new ExpressionTransforms() // for RemoveDeadVariableInit
 					}
 				},
 				// RemoveDeadVariableInit must run after ExpressionTransforms so that stobj(ldloca V, ...)

ICSharpCode.Decompiler/CSharp/Transforms/IntroduceUsingDeclarations.cs

@@ -40,8 +40,6 @@ namespace ICSharpCode.Decompiler.CSharp.Transforms
 			var requiredImports = new FindRequiredImports(context);
 			compilationUnit.AcceptVisitor(requiredImports);
 			
-			requiredImports.ImportedNamespaces.Add("System"); // always import System, even when not necessary
-			
 			var usingScope = new UsingScope();
 
 			var insertionPoint = compilationUnit.Children.LastOrDefault(n => n is PreProcessorDirective p && p.Type == PreProcessorDirectiveType.Define);