Remove export_lfs_objects_projects feature flag

The flag has been enabled by default for 1 year, so will be safe to
remove.

Flag was added in d18ee3fa as part of
https://gitlab.com/gitlab-org/gitlab/-/issues/11090.

config/feature_flags/development/export_lfs_objects_projects.yml

----
-name: export_lfs_objects_projects
-introduced_by_url: 
-rollout_issue_url: 
-group: 
-type: development
-default_enabled: true

lib/gitlab/import_export/lfs_saver.rb

@@ -21,10 +21,10 @@ module Gitlab
             save_lfs_object(lfs_object)
           end
 
-          append_lfs_json_for_batch(batch) if write_lfs_json_enabled?
+          append_lfs_json_for_batch(batch)
         end
 
-        write_lfs_json if write_lfs_json_enabled?
+        write_lfs_json
 
         true
       rescue => e
@@ -35,10 +35,6 @@ module Gitlab
 
       private
 
-      def write_lfs_json_enabled?
-        ::Feature.enabled?(:export_lfs_objects_projects, default_enabled: true)
-      end
-
       def save_lfs_object(lfs_object)
         if lfs_object.local_store?
           copy_file_for_lfs_object(lfs_object)

spec/lib/gitlab/import_export/lfs_saver_spec.rb

@@ -74,14 +74,6 @@ RSpec.describe Gitlab::ImportExport::LfsSaver do
             }
           )
         end
-
-        it 'does not save a json file if feature is disabled' do
-          stub_feature_flags(export_lfs_objects_projects: false)
-
-          saver.save
-
-          expect(File.exist?(lfs_json_file)).to eq(false)
-        end
       end
     end