Delete unused cluster health tab UI code

What does this MR do and why?

The cluster health tab was removed as part of the %16.0 removals.

This MR deletes the now-inactive frontend code.

issue: #393926 (closed)

Screenshots or screen recordings

No change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Apply the following diff to disable the feature flag and add a live debug point:

   Click to expand

   diff --git a/spec/features/clusters/cluster_detail_page_spec.rb b/spec/features/clusters/cluster_detail_page_spec.rb
   index 31dec5e38daf..be92fde08d4d 100644
   --- a/spec/features/clusters/cluster_detail_page_spec.rb
   +++ b/spec/features/clusters/cluster_detail_page_spec.rb
   @@ -11,6 +11,7 @@
   
      before do
        sign_in(current_user)
   +    stub_feature_flags(remove_monitor_metrics: false)
      end
   
      shared_examples 'show page' do
   @@ -23,6 +24,8 @@
        it 'allow the user to set domain', :js do
          visit cluster_path
   
   +      live_debug
   +
          within '.js-cluster-details-form' do
            fill_in('cluster_base_domain', with: 'test.com')
            click_on 'Save changes'

2. Run the spec: bin/rspec spec/features/clusters/cluster_detail_page_spec.rb.

3. When the browser window opens, use the command line user/password to log in and see the page.

4. Observe that the 'Health' tab is removed from the cluster page and no errors are present.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

locale/gitlab.pot

@@ -10721,9 +10721,6 @@ msgstr ""
 msgid "ClusterIntegration|If you do not wish to delete all associated GitLab resources, you can simply remove the integration."
 msgstr ""
-msgid "ClusterIntegration|In order to view the health of your cluster, you must first enable Prometheus in the Integrations tab."
-msgstr ""
-
 msgid "ClusterIntegration|Instance cluster"
 msgstr ""