Fix TypeErasure not removing all nullability annotations.

ICSharpCode.Decompiler/TypeSystem/NormalizeTypeVisitor.cs

@@ -21,16 +21,6 @@ namespace ICSharpCode.Decompiler.TypeSystem
 			RemoveNullability = true,
 		};
 
-		internal static readonly NormalizeTypeVisitor RemoveModifiersAndNullability = new NormalizeTypeVisitor {
-			ReplaceClassTypeParametersWithDummy = false,
-			ReplaceMethodTypeParametersWithDummy = false,
-			DynamicAndObject = false,
-			TupleToUnderlyingType = false,
-			RemoveModOpt = true,
-			RemoveModReq = true,
-			RemoveNullability = true,
-		};
-
 		public bool EquivalentTypes(IType a, IType b)
 		{
 			a = a.AcceptVisitor(this);
@@ -52,6 +42,8 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				return DummyTypeParameter.GetMethodTypeParameter(type.Index);
 			} else if (type.OwnerType == SymbolKind.TypeDefinition && ReplaceClassTypeParametersWithDummy) {
 				return DummyTypeParameter.GetClassTypeParameter(type.Index);
+			} else if (RemoveNullability && type is NullabilityAnnotatedTypeParameter natp) {
+				return natp.TypeWithoutAnnotation.AcceptVisitor(this);
 			} else {
 				return base.VisitTypeParameter(type);
 			}
@@ -82,7 +74,7 @@ namespace ICSharpCode.Decompiler.TypeSystem
 		public override IType VisitNullabilityAnnotatedType(NullabilityAnnotatedType type)
 		{
 			if (RemoveNullability)
-				return base.VisitNullabilityAnnotatedType(type).ChangeNullability(Nullability.Oblivious);
+				return type.TypeWithoutAnnotation.AcceptVisitor(this);
 			else
 				return base.VisitNullabilityAnnotatedType(type);
 		}