[System.ComponentModel.DataAnnotations] Temporarily bring back the original AssociatedMetadataTypeTypeDescriptor

To fix the failing `MonoTests.System.Web.DynamicData.MetaColumnTest` tests (specifically those that use the `FooWithMetadataType` class) I needed to bring back the original `AssociatedMetadataTypeTypeDescriptor` and remove the one from MS referencesource.

A quick investigation showed that the returned attributes for the `FooWithMetadataType` only include the attributes from `FooMetaDataType` and not from `FooWithMetadataType` merged with `FooMetaDataType`.

This obviously needs more investigation, but bringing back the original implementation fixes the tests for now.

*edit* this got split up as per https://github.com/mono/mono/pull/1440#issuecomment-65658977