Remove the use_workhorse_s3_client feature flag

This has been present since 13.2. Time to remove!

Changelog: removed

config/feature_flags/development/use_workhorse_s3_client.yml

----
-name: use_workhorse_s3_client
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35480
-rollout_issue_url: 
-milestone: '13.2'
-type: development
-group: group::source code
-default_enabled: true

lib/object_storage/direct_upload.rb

@@ -112,7 +112,6 @@ module ObjectStorage
     end
 
     def use_workhorse_s3_client?
-      return false unless Feature.enabled?(:use_workhorse_s3_client, default_enabled: true)
       return false unless config.use_iam_profile? || config.consolidated_settings?
       # The Golang AWS SDK does not support V2 signatures
       return false unless credentials.fetch(:aws_signature_version, 4).to_i >= 4

spec/lib/object_storage/direct_upload_spec.rb

@@ -136,16 +136,6 @@ RSpec.describe ObjectStorage::DirectUpload do
         end
       end
 
-      context 'when feature flag is disabled' do
-        before do
-          stub_feature_flags(use_workhorse_s3_client: false)
-        end
-
-        it 'does not enable Workhorse client' do
-          expect(subject[:UseWorkhorseClient]).to be false
-        end
-      end
-
       context 'when V2 signatures are used' do
         before do
           credentials[:aws_signature_version] = 2