Fixed System.Web.dynamicData tests

Some tests tested the wrong exception now that the System.ComponentModel.DataAnnotations code from MS is in (see commit d0adcb170043768de8f4849737ae9c343a8b17b2).

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` (see commit 7c4c05a6eb6ea8645a83a80e4ecb827120f1f93f) 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.