Remove BuildsEmailService records from services

The `BuildsEmailService` records were migrated to
`PipelinesEmailService` in 12dd5ac2.

That commit also deleted the corresponding spec and added a comment that
this model could be removed in 9.1.

However, there are still 305 `BuildsEmailService` records in the
GitLab.com database.

This change:

1. Deletes records in the `services` table with type of
`"BuildsEmailService"`.
2. Deletes the model

https://gitlab.com/gitlab-org/gitlab/-/issues/331064
https://gitlab.com/gitlab-org/gitlab/-/issues/332551

Changelog: removed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63264

app/models/integrations/builds_email.rb

-# frozen_string_literal: true
-
-# This class is to be removed with 9.1
-# We should also by then remove BuildsEmailService from database
-# https://gitlab.com/gitlab-org/gitlab/-/issues/331064
-module Integrations
-  class BuildsEmail < Integration
-    def self.to_param
-      'builds_email'
-    end
-
-    def self.supported_events
-      %w[]
-    end
-  end
-end

db/migrate/20210603222333_remove_builds_email_service_from_services.rb

+# frozen_string_literal: true
+
+class RemoveBuildsEmailServiceFromServices < ActiveRecord::Migration[6.1]
+  def up
+    execute("DELETE from services WHERE type = 'BuildsEmailService'")
+  end
+
+  def down
+    # no-op
+  end
+end

db/schema_migrations/20210603222333

+fb02e0fee2760dad203b54d81c342dbf1461b3010503cab05da1eb14ab5d33da
\ No newline at end of file

spec/migrations/20210603222333_remove_builds_email_service_from_services_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+require_migration!('remove_builds_email_service_from_services')
+
+RSpec.describe RemoveBuildsEmailServiceFromServices do
+  let(:namespaces) { table(:namespaces) }
+  let(:projects) { table(:projects) }
+  let(:services) { table(:services) }
+  let(:namespace) { namespaces.create!(name: 'foo', path: 'bar') }
+  let(:project) { projects.create!(namespace_id: namespace.id) }
+
+  it 'correctly deletes `BuildsEmailService` services' do
+    services.create!(project_id: project.id, type: 'BuildsEmailService')
+    services.create!(project_id: project.id, type: 'OtherService')
+
+    expect(services.all.pluck(:type)).to match_array %w[BuildsEmailService OtherService]
+
+    migrate!
+
+    expect(services.all.pluck(:type)).to eq %w[OtherService]
+  end
+end