Remove GitlabIssueTrackerService records

The GitlabIssueTrackerService class is not used anymore
and will be removed too.

changelogs/unreleased/218526_backstage_remove_gitlab_issue_tracker_service_records.yml

+---
+title: Clean up GitlabIssueTrackerService database records
+merge_request: 35221
+author:
+type: other

db/post_migrate/20200623142159_remove_gitlab_issue_tracker_service_records.rb

+# frozen_string_literal: true
+
+class RemoveGitlabIssueTrackerServiceRecords < 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/structure.sql

@@ -23535,6 +23535,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200623000320
 20200623121135
 20200623141544
+20200623142159
 20200623170000
 20200623185440
 20200624075411

spec/migrations/remove_gitlab_issue_tracker_service_records_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require Rails.root.join('db', 'post_migrate', '20200623142159_remove_gitlab_issue_tracker_service_records.rb')
+
+RSpec.describe RemoveGitlabIssueTrackerServiceRecords do
+  let(:services) { table(:services) }
+
+  before do
+    5.times { services.create!(type: 'GitlabIssueTrackerService') }
+    services.create!(type: 'SomeOtherType')
+  end
+
+  it 'removes services records of type GitlabIssueTrackerService', :aggregate_failures do
+    expect { migrate! }.to change { services.count }.from(6).to(1)
+    expect(services.first.type).to eq('SomeOtherType')
+    expect(services.where(type: 'GitlabIssueTrackerService')).to be_empty
+  end
+end