Revert "Fixes make dist"

This reverts commit 87e95b46ed84d290d1e26806881a9a03e5864633.

This puts DataContractJsonSerializerSettings.cs back into
mcs/class/System.ServiceModel.Web/System.Runtime.Serialization.Json.

Let's please not have a single file from this namespace in another
directory, either move the entire System.Runtime.Serialization.Json
stuff into System.Runtime.Serialization or leave it here.