Mark unused code as such to satisfy -D warnings in 1.78.0 (#3624)

## Motivation and Context
Mark unused code as such for [the latest
compiler](https://blog.rust-lang.org/2024/05/02/Rust-1.78.0.html).

## Description
We use the latest compiler as part of our release of AWS SDKs, which
ensures that 3rd party dependencies should build even if they release a
new version requiring a newer MSRV than we specify within this
repository. At the time of writing, the latest compiler is 1.78.0, and
it is now able to detect more unused code patterns that our current MSRV
1.75.0.

The compiler 1.78.0 unfortunately caused a build failure in
[check-sdk-codegen-unit-tests](https://github.com/smithy-lang/smithy-rs/blob/e8449bd152ffd6471516a125f565c63dd690c034/tools/ci-scripts/check-sdk-codegen-unit-tests)
as follows
```
src/smithy-rs/aws/rust-runtime/aws-runtime/src/env_config/section.rs:21:8
       |
    16 | pub(crate) trait Section {
       |                  ------- methods in this trait
    ...
    21 |     fn properties(&self) -> &HashMap<String, Property>;
       |        ^^^^^^^^^^
    ...
    27 |     fn is_empty(&self) -> bool;
       |        ^^^^^^^^
       |
       = note: `-D dead-code` implied by `-D warnings`
       = help: to override `-D warnings` add `#[allow(dead_code)]
```
We cannot eliminate this failure even if we specify `RUSTFLAGS="-D
warnings"` because the generated client tests will render the following
in its dot cargo directory:
```
cat .cargo/config.toml
[build]
rustflags = ["--deny","warnings"]
```
Plus, trying to eliminate `--deny warnings` contradicts what
https://github.com/smithy-lang/smithy-rs/issues/3194 is trying to
achieve. To fix it, we will follow what the compiler suggests to fix.

## Testing
Verified it fixed our internal release pipeline.

----

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