Remove feature flag clusters_list_redesign

What does this MR do?

The current Cluster Index View has the following code:

- if Feature.enabled?(:clusters_list_redesign, default_enabled: true)
      #js-clusters-list-app{ data: { endpoint: clusterable.index_path(format: :json) } }
    - else
      .clusters-table.js-clusters-list
        .gl-responsive-table-row.table-row-header{ role: "row" }
          .table-section.section-60{ role: "rowheader" }
            = s_("ClusterIntegration|Kubernetes cluster")
          .table-section.section-30{ role: "rowheader" }
            = s_("ClusterIntegration|Environment scope")
          .table-section.section-10{ role: "rowheader" }
        - @clusters.each do |cluster|
          = render "cluster", cluster: cluster.present(current_user: current_user)
      = paginate @clusters, theme: "gitlab"

Feature.enabled?(:clusters_list_redesign, default_enabled: true) is part of a redesign where we:

• Refactor/Move the cluster list from haml to vue
• Add additional columns to the cluster table. (See related Epic: &2666 (closed))

The feature has been enabled deployed with no bugs. We are no now ready to remove the feature flag.

This MR will:

• Remove feature flag clusters_list_redesign
• Remove haml and helper methods related to the old cluster table design
• Remove related tests
• Remove related translations

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #220182 (closed)