Revert post-deploy migration

This has failed in a GitLab.com deploy:
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/2802

db/post_migrate/20200914185610_schedule_sync_blocking_issues_count.rb

-# frozen_string_literal: true
-
-class ScheduleSyncBlockingIssuesCount < ActiveRecord::Migration[6.0]
-  include Gitlab::Database::MigrationHelpers
-
-  BATCH_SIZE = 50
-  DELAY_INTERVAL = 120.seconds.to_i
-  MIGRATION = 'SyncBlockingIssuesCount'.freeze
-
-  disable_ddl_transaction!
-
-  class Issue < ActiveRecord::Base
-    include EachBatch
-
-    self.table_name = 'issues'
-  end
-
-  def up
-    return unless Gitlab.ee?
-
-    blocking_issues_ids = <<-SQL
-      SELECT issue_links.source_id AS blocking_issue_id
-      FROM issue_links
-      INNER JOIN issues ON issue_links.source_id = issues.id
-      WHERE issue_links.link_type = 1
-      AND issues.state_id = 1
-      AND issues.blocking_issues_count = 0
-      UNION
-      SELECT issue_links.target_id AS blocking_issue_id
-      FROM issue_links
-      INNER JOIN issues ON issue_links.target_id = issues.id
-      WHERE issue_links.link_type = 2
-      AND issues.state_id = 1
-      AND issues.blocking_issues_count = 0
-    SQL
-
-    relation =
-      Issue.where("id IN(#{blocking_issues_ids})") # rubocop:disable GitlabSecurity/SqlInjection
-
-    queue_background_migration_jobs_by_range_at_intervals(
-      relation,
-      MIGRATION,
-      DELAY_INTERVAL,
-      batch_size: BATCH_SIZE
-    )
-  end
-
-  def down
-    # no-op
-  end
-end

db/schema_migrations/20200914185610

-50cdb8d42baa0890b39ca7d07a4c157aea7d52a39dee48ee59e51417442eaaf4
\ No newline at end of file

ee/spec/migrations/schedule_sync_blocking_issues_count_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-require Rails.root.join('db', 'post_migrate', '20200914185610_schedule_sync_blocking_issues_count')
-
-RSpec.describe ScheduleSyncBlockingIssuesCount do
-  let(:issues) { table(:issues) }
-  let(:links) { table(:issue_links) }
-  let(:migration) { described_class::MIGRATION }
-
-  # Blocking issues
-  let!(:issue_1) { issues.create!(description: 'blocking 1', state_id: 1) }
-  let!(:issue_2) { issues.create!(description: 'blocking 2', state_id: 1) }
-  let!(:issue_3) { issues.create!(description: 'blocking 3', state_id: 2) }
-  let!(:issue_4) { issues.create!(description: 'blocking 4', state_id: 1 ) }
-
-  # Blocked issues
-  let!(:issue_5) { issues.create!(description: 'blocked 1', state_id: 1) }
-  let!(:issue_6) { issues.create!(description: 'blocked 2', state_id: 1) }
-  let!(:issue_7) { issues.create!(description: 'blocked 3', state_id: 1) }
-  let!(:issue_8) { issues.create!(description: 'blocked 4', state_id: 1) }
-  let!(:issue_9) { issues.create!(description: 'blocked 5', state_id: 1) }
-  let!(:issue_10) { issues.create!(description: 'blocked 6', state_id: 1) }
-
-  before do
-    stub_const("#{described_class.name}::BATCH_SIZE", 2)
-
-    # Issue links
-    # -----------
-    # issue_1 blocks two issues with link_type BLOCKS
-    # issue_2 blocks two issues with link_type IS_BLOCKED_BY
-    # issue_3 blocks one issue with link_type IS_BLOCKED_BY but it is closed
-    # issue_4 blocks one issue with link_type BLOCKS
-    links.create!(link_type: 1, source_id: issue_1.id, target_id: issue_5.id)
-    links.create!(link_type: 1, source_id: issue_1.id, target_id: issue_6.id)
-    links.create!(link_type: 2, source_id: issue_7.id, target_id: issue_2.id)
-    links.create!(link_type: 2, source_id: issue_8.id, target_id: issue_2.id)
-    links.create!(link_type: 1, source_id: issue_4.id, target_id: issue_9.id)
-    links.create!(link_type: 2, source_id: issue_10.id, target_id: issue_3.id)
-  end
-
-  context 'scheduling migrations' do
-    before do
-      Sidekiq::Worker.clear_all
-    end
-
-    it 'correctly schedules issuable sync background migration' do
-      Sidekiq::Testing.fake! do
-        freeze_time do
-          migrate!
-
-          expect(migration).to be_scheduled_delayed_migration(120.seconds, issue_1.id, issue_2.id)
-          expect(migration).to be_scheduled_delayed_migration(240.seconds, issue_4.id, issue_4.id)
-          expect(BackgroundMigrationWorker.jobs.size).to eq(2)
-        end
-      end
-    end
-  end
-
-  context 'running background migration' do
-    it 'correctly populates issues blocking_issues_count', :sidekiq_might_not_need_inline do
-      migrate!
-
-      expect(issue_1.reload.blocking_issues_count).to eq(2)
-      expect(issue_2.reload.blocking_issues_count).to eq(2)
-      expect(issue_3.reload.blocking_issues_count).to eq(0)
-      expect(issue_4.reload.blocking_issues_count).to eq(1)
-    end
-  end
-end