Remove Gitlab::BackgroundMigration exxxplicit namespaces from migration classes

lib/gitlab/background_migration/fill_file_store_job_artifact.rb

@@ -10,8 +10,8 @@ module Gitlab
       end
 
       def perform(start_id, stop_id)
-        Gitlab::BackgroundMigration::FillFileStoreJobArtifact::JobArtifact
-          .where('file_store is NULL')
+        FillFileStoreJobArtifact::JobArtifact
+          .where(file_store: nil)
           .where(id: (start_id..stop_id))
           .update_all(file_store: 1)
       end

lib/gitlab/background_migration/fill_file_store_lfs_object.rb

@@ -10,8 +10,8 @@ module Gitlab
       end
 
       def perform(start_id, stop_id)
-        Gitlab::BackgroundMigration::FillFileStoreLfsObject::LfsObject
-          .where('file_store is NULL')
+        FillFileStoreLfsObject::LfsObject
+          .where(file_store: nil)
           .where(id: (start_id..stop_id))
           .update_all(file_store: 1)
       end

lib/gitlab/background_migration/fill_store_upload.rb

@@ -11,8 +11,8 @@ module Gitlab
       end
 
       def perform(start_id, stop_id)
-        Gitlab::BackgroundMigration::FillStoreUpload::Upload
-          .where('store is NULL')
+        FillStoreUpload::Upload
+          .where(store: nil)
           .where(id: (start_id..stop_id))
           .update_all(store: 1)
       end