Remove `create-session-mode` header for internally used CreateSession API in S3 Express (#3992)

## Motivation and Context
This PR stops populating `SessionMode` by default for the
`CreateSession` API internally used by S3 Express, as the value already
matched the service-side default and it was already not sent by other
SDKs.

## Testing
- Existing tests in CI (i.e. canary)
- Updated S3 Express integration tests to verify that no `SessionMode`
header is present by the `CreateSession` API internally used by S3
Express

## Checklist
- [x] For changes to the AWS SDK, generated SDK code, or SDK runtime
crates, I have created a changelog entry Markdown file in the
`.changelog` directory, specifying "aws-sdk-rust" in the `applies_to`
key.

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