fix: Remove assertion on HttpApiKey scheme being present in client co… (#4309)

## Description
- Removes erroneous assertion, further explained in
https://github.com/smithy-lang/smithy-rs/issues/4226
- Closes https://github.com/smithy-lang/smithy-rs/issues/4226




## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] For changes to the smithy-rs codegen or runtime crates, I have
created a changelog entry Markdown file in the `.changelog` directory,
specifying "client," "server," or both in the `applies_to` key.

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Co-authored-by: Aaron Todd <aajtodd@users.noreply.github.com>
Co-authored-by: Landon James <lnj@amazon.com>