Remove create-session-mode header for internally used CreateSession API in S3 Express

[ysaito1001]

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

• 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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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

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[github-actions]

A new generated diff is ready to view.

• AWS SDK (ignoring whitespace)
• No codegen difference in the Client Test
• No codegen difference in the Server Test
• No codegen difference in the Server Test Python
• No codegen difference in the Server Test Typescript

A new doc preview is ready to view.

[landonxjames]

LGTM