Remove `once_cell` from public API (#2973)

## Motivation and Context
Addresses  7 in #2413 

## Description
This small PR removes uses of `once_cell` from public API. The
`http_versions` module in the `aws_smithy_http` crate was dead-code.

## Testing
Relies on existing tests in CI.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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Co-authored-by: ysaito1001 <awsaito@amazon.com>