Remove flags from release (#3521)

## Motivation and Context
This removes the (unneeded) flags from the smithy-rs release job that
set the stable/unstable versions.

## Testing
- [x] ran a dry run on the branch

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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