Remove remaining middleware code (#3038)

This PR removes the remaining rust-runtime code to support middleware,
deprecates empty crates, and removes the remaining codegen references to
any of that code. In the interest of keeping code review easier, a
separate PR will finish addressing the remaining
`TODO(enableNewSmithyRuntime)` comments.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._