Remove release-environment downstream pipeline from canonical

What does this MR do and why?

We currently deploy to release-environment (RE, https://gitlab.com/gitlab-com/gl-infra/release-environments) on both the canonical and security mirror's stable branches supported by the current maintenance policy.

We mirror all the commits from canonical to security, and during patch release processes, the security mirror's stable branches would have more commits. Currently this means that the same commit from the canonical stable branch gets deployed to RE twice, once from canonical stable branch, and another time from the security mirror's stable branch.

In order to improve cost efficiency (cost breakdown in gitlab-com/gl-infra/delivery#20202 (closed)), we should just deploy to RE from the security stable branches.

This MR's to remove the downstream integration to RE from canonical stable branches.

There will be a followup MR in https://gitlab.com/gitlab-com/gl-infra/release-environments to actually spin down the non-security deployments.

Addresses: gitlab-com/gl-infra/delivery#20202 (closed)

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