[smithy-rs] Remove aws-smithy-runtime-test from 0.55.x (#2586)

## Motivation and Context
The `aws-smithy-runtime-test` crate has repeatedly caused an issue each
time we released `aws-sdk-rust` from the `smithy-rs-release-0.55.x`
branch. Example PRs for fixing the issue in `aws-sdk-rust` include
- #778
- #787
- #789

This PR will remove the crate in question from the release branch so
that next time we release from `smithy-rs-release-0.55.x`, we do not
need to deal with what's listed above.

The issue only occurs in the `smithy-rs-release-0.55.x` branch, and the
main branch, as of this writing, no longer has it because the crate has
moved from `aws/sdk/integration-tests` to `aws/sra-test`.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

Co-authored-by: Yuki Saito <awsaito@amazon.com>