Curation: Delete examples for obsolete types

[gewarren]

Some APIs, like XmlValidatingReader, have been obsolete since .NET Framework 2.0. We should delete all example text and code for these APIs.

Rubric is delete code examples for all members if the parent type is:

• Obsolete since .NET Framework 4 or earlier
• Obsolete in .NET and deprecated in .NET Fx (e.g. CAS)
• Obsolete in .NET and not editor browsable in .NET Fx