Remove lfs_link_existing_object feature flag

lfs_link_existing_object is enabled for a while without issues
we can remove it

app/services/projects/lfs_pointers/lfs_download_service.rb

@@ -22,7 +22,7 @@ module Projects
       def execute
         return unless project&.lfs_enabled? && lfs_download_object
         return error("LFS file with oid #{lfs_oid} has invalid attributes") unless lfs_download_object.valid?
-        return link_existing_lfs_object! if Feature.enabled?(:lfs_link_existing_object, project, default_enabled: true) && lfs_size > LARGE_FILE_SIZE && lfs_object
+        return link_existing_lfs_object! if lfs_size > LARGE_FILE_SIZE && lfs_object
 
         wrap_download_errors do
           download_lfs_file!

config/feature_flags/development/lfs_link_existing_object.yml

----
-name: lfs_link_existing_object
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/41770
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/249246
-group: group::source code
-type: development
-default_enabled: true

spec/services/projects/lfs_pointers/lfs_download_service_spec.rb

@@ -241,18 +241,6 @@ RSpec.describe Projects::LfsPointers::LfsDownloadService do
       context 'and first fragments are the same' do
         let(:lfs_content) { existing_lfs_object.file.read }
 
-        context 'when lfs_link_existing_object feature flag disabled' do
-          before do
-            stub_feature_flags(lfs_link_existing_object: false)
-          end
-
-          it 'does not call link_existing_lfs_object!' do
-            expect(subject).not_to receive(:link_existing_lfs_object!)
-
-            subject.execute
-          end
-        end
-
         it 'returns success' do
           expect(subject.execute).to eq({ status: :success })
         end