Early migrations populating fork-networks: no-op

Since populating the fork networks was scheduled multiple times
because of bugs that needed to be fixed:

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15595/
  Creating fork networks for projects that were missing the root of
  the fork network.

- https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15709
  The API allowed creating forked_project_links without
  fork_network_members.

Scheduling this migration multiple times would case it to run
concurrently. Which in turn would try to insert the same data into
`fork_network_members` causing duplicate key errors.

This avoids running the migration multiple times.

changelogs/unreleased/bvl-fix-concurrent-fork-network-migrations.yml

+---
+title: Avoid running `PopulateForkNetworksRange`-migration multiple times
+merge_request: 16988
+author:
+type: fixed

db/migrate/20170929131201_populate_fork_networks.rb

@@ -6,22 +6,8 @@ class PopulateForkNetworks < ActiveRecord::Migration
   DOWNTIME = false
-  MIGRATION = 'PopulateForkNetworksRange'.freeze
-  BATCH_SIZE = 100
-  DELAY_INTERVAL = 15.seconds
-
-  disable_ddl_transaction!
-
-  class ForkedProjectLink < ActiveRecord::Base
-    include EachBatch
-
-    self.table_name = 'forked_project_links'
-  end
-
   def up
-    say 'Populating the `fork_networks` based on existing `forked_project_links`'
-
-    queue_background_migration_jobs_by_range_at_intervals(ForkedProjectLink, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+    say 'Fork networks will be populated in 20171205190711 - RescheduleForkNetworkCreationCaller'
   end
   def down

db/post_migrate/20171124150326_reschedule_fork_network_creation.rb

@@ -3,22 +3,8 @@ class RescheduleForkNetworkCreation < ActiveRecord::Migration
   DOWNTIME = false
-  MIGRATION = 'PopulateForkNetworksRange'.freeze
-  BATCH_SIZE = 100
-  DELAY_INTERVAL = 15.seconds
-
-  disable_ddl_transaction!
-
-  class ForkedProjectLink < ActiveRecord::Base
-    include EachBatch
-
-    self.table_name = 'forked_project_links'
-  end
-
   def up
-    say 'Populating the `fork_networks` based on existing `forked_project_links`'
-
-    queue_background_migration_jobs_by_range_at_intervals(ForkedProjectLink, MIGRATION, DELAY_INTERVAL, batch_size: BATCH_SIZE)
+    say 'Fork networks will be populated in 20171205190711 - RescheduleForkNetworkCreationCaller'
   end
   def down