Remove the deploy_version dependency from the post-deployment classes

Once the post-migration pipeline is set, we should be able to remove the deploy_version dependency from the post-deployment notification classes

For context, the post-migration pipeline is independent of a deploy version, when executed, it doesn't install any package it just executes the pending post-migrations. To keep compatibility with the coordinated pipeline, the slack classes were modified to consider both pipelines: the coordinated pipeline that uses a deploy_version and the independent post-deploy pipeline that doesn't require a deploy version, see gitlab-org/release-tools!1758 (diffs). We shouldn't need this compatibility anymore, so it should be fine to remove the deploy_version dependency.