Remove AI Tool Reduction experimental APIs

[jeffhandley]

Many of the emerging approaches operate at the agent level rather than the chat client level, so it's unclear that IChatClient middleware is even the right layer for this. Either way, I don't think it makes sense to commit to a first-class MEAI abstraction in this PR. This can still be implemented as custom middleware by anyone who needs it today 🙂

Originally posted by @MackinnonBuck in #6865 (comment)

Per this conclusion, we should remove the experimental APIs related to tool reduction.