Remove feature flag for auto-linking LFS objects in forks

The `lfs_auto_link_fork_source` feature flag was shipped in GitLab
14.6 via https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75972
and enabled by default in GitLab 14.7.

This feature automatically links LFS objects to a fork if the parent
already has the same object, eliminating the need for the client to
transfer the files again.

Since this has been working well in production for two months,
drop the feature flag.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/348243

Changelog: changed

app/controllers/repositories/lfs_api_controller.rb

@@ -155,7 +155,6 @@ module Repositories
     end
 
     def should_auto_link?
-      return false unless Feature.enabled?(:lfs_auto_link_fork_source, project, default_enabled: :yaml)
       return false unless project.forked?
 
       # Sanity check in case for some reason the user doesn't have access to the parent

config/feature_flags/development/lfs_auto_link_fork_source.yml

----
-name: lfs_auto_link_fork_source
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75972
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/348243
-milestone: '14.6'
-type: development
-group: group::source code
-default_enabled: true

spec/requests/lfs_http_spec.rb

@@ -546,14 +546,6 @@ RSpec.describe 'Git LFS API and storage' do
 
                       expect(lfs_object.reload.projects.pluck(:id)).to match_array([other_project.id, project.id])
                     end
-
-                    context 'when feature flag is disabled' do
-                      before do
-                        stub_feature_flags(lfs_auto_link_fork_source: false)
-                      end
-
-                      it_behaves_like 'batch upload with existing LFS object'
-                    end
                   end
 
                   context 'when user does not have access to parent' do