Stop `cargo check` examples in smithy-rs CI (#4063)

## Motivation and Context
Removes a CI step that runs `cargo check` against the examples from the
`aws-doc-sdk-examples` repository

## Description
We've long had the said CI step due to the churn in the public API prior
to GA. However, value of doing so has diminished after GA, especially
when we have a separate check for semver hazards.

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