Remove a dead step from release workflow (#3580)

## Motivation and Context
A step `trim-changelog-next-on-main` has been
[skipped](https://github.com/smithy-lang/smithy-rs/actions/workflows/release.yml?query=actor%3Aaws-sdk-rust-ci)
probably since a release branch `smithy-rs-release-1.x.y` was created.

## Description
We no longer directly remove released entries from CHANGELOG.next.toml
on `main`. We do so by backport PRs to merge `smithy-rs-release-1.x.y`
to `main`. So `open-backport-pull-request` now depends on `release`,
instead of `trim-changelog-next-on-main`.

## Testing
`open-backport-pull-request` depending on `release` can only be "tested"
in prod runs, so we'll see what happens in the next release. If
`open-backport-pull-request` fails, we will manually run it and address
whatever issue that comes up.

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