Revert tilde version requirements for runtime crates (#3122)

## Motivation and Context
Now that we have adopted the approach of distinguishing between stable
and unstable runtime crates, the tilde version requirements introduced
earlier is no longer necessary.

## Description
Reverts the following PRs:
https://github.com/awslabs/smithy-rs/pull/3009
https://github.com/awslabs/smithy-rs/pull/3025

## Testing
Relied on the existing tests (if we can generate SDKs in CI, we should
be good to go)

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