Remove again GitlabIssueTrackerService records

This is the last commit of the plan to remove the
GitlabIssueTrackerService, see:
https://gitlab.com/gitlab-org/gitlab/-/issues/218526#note_380963641

1. Remove the records in a post-migration
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/35221
2. Remove the GitLabIssueTrackerService class
   https://gitlab.com/gitlab-org/gitlab/-/merge_requests/36063
3. Run the migration again to remove any existing records in a
   post-migration. This commit!

changelogs/unreleased/218526-remove-again-gitlab-issue-tracker-service.yml

+---
+title: Remove GitlabIssueTrackerService database records
+merge_request: 37931
+author:
+type: other

db/post_migrate/20200727100631_remove_again_gitlab_issue_tracker_service_records.rb

+# frozen_string_literal: true
+
+class RemoveAgainGitlabIssueTrackerServiceRecords < ActiveRecord::Migration[6.0]
+  DOWNTIME = false
+  BATCH_SIZE = 5000
+
+  disable_ddl_transaction!
+
+  class Service < ActiveRecord::Base
+    include EachBatch
+
+    self.table_name = 'services'
+
+    def self.gitlab_issue_tracker_service
+      where(type: 'GitlabIssueTrackerService')
+    end
+  end
+
+  def up
+    Service.each_batch(of: BATCH_SIZE) do |services|
+      services.gitlab_issue_tracker_service.delete_all
+    end
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20200727100631

+83c1dca01d4e56c22f6c1cda249c9b162eb7c31e2b164629bf51ea9aa9dd8fb5
\ No newline at end of file