Drop workhorse_google_client feature flag

Dropping this feature flag causes all Workhorse-enabled uploads to use
GoCloud's Google client instead of pre-signed URLs. This not only
improves upload reliability and performance, but also adds support for
Customer-Managed Encryption Keys (CMEK).

Relates to:

* #372596
* #441782

Changelog: changed

config/feature_flags/gitlab_com_derisk/workhorse_google_client.yml

----
-name: workhorse_google_client
-feature_issue_url: https://gitlab.com/groups/gitlab-org/-/epics/9457
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/96891
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/372596
-milestone: '15.6'
-group: group::package registry
-type: gitlab_com_derisk
-default_enabled: false

lib/object_storage/direct_upload.rb

@@ -68,7 +68,7 @@ def workhorse_client_hash
         workhorse_aws_hash
       elsif config.azure?
         workhorse_azure_hash
-      elsif Feature.enabled?(:workhorse_google_client, Feature.current_request) && config.google?
+      elsif config.google?
         workhorse_google_hash
       else
         {}

spec/lib/object_storage/direct_upload_spec.rb

@@ -205,16 +205,6 @@
           expect(subject[:ObjectStorage][:GoCloudConfig]).to eq({ URL: gocloud_url })
         end
       end
-
-      context 'with workhorse_google_client disabled' do
-        before do
-          stub_feature_flags(workhorse_google_client: false)
-        end
-
-        it 'does not set Workhorse client data' do
-          expect(subject.keys).not_to include(:UseWorkhorseClient, :RemoteTempObjectID, :ObjectStorage)
-        end
-      end
     end
 
     shared_examples 'a valid AzureRM upload' do