Remove RequiresUnreferencedCode from DefaultValueAttribute

This constructor of `DefaultValue` has [RUC ](https://github.com/dotnet/runtime/blob/36a2a8dce86dd8de836d53e293531ad55b61eb88/src/libraries/System.Private.CoreLib/src/System/ComponentModel/DefaultValueAttribute.cs#L30)on it. This is likely due to a concern round converters with generic types (Nullable<T>) which has been since fixed in the trimmer.
