Remove db/migrate/20170828135939_migrate_user_external_mail_data.rb

This file is a duplicate of a post-deploy migration and appears to have
been left in by mistake.

Looping through the Users table in a foreground migration would've been
a bad idea.

[ci skip]

db/migrate/20170828135939_migrate_user_external_mail_data.rb

-# See http://doc.gitlab.com/ce/development/migration_style_guide.html
-# for more information on how to write migrations for GitLab.
-
-class MigrateUserExternalMailData < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
-  DOWNTIME = false
-
-  disable_ddl_transaction!
-
-  class User < ActiveRecord::Base
-    self.table_name = 'users'
-
-    include EachBatch
-  end
-
-  class UserSyncedAttributesMetadata < ActiveRecord::Base
-    self.table_name = 'user_synced_attributes_metadata'
-
-    include EachBatch
-  end
-
-  def up
-    User.each_batch do |batch|
-      start_id, end_id = batch.pluck('MIN(id), MAX(id)').first
-
-      execute <<-EOF
-        INSERT INTO user_synced_attributes_metadata (user_id, provider, email_synced)
-        SELECT id, email_provider, external_email
-        FROM users
-        WHERE external_email = TRUE
-        AND NOT EXISTS (
-          SELECT true
-          FROM user_synced_attributes_metadata
-          WHERE user_id = users.id
-          AND provider = users.email_provider
-        )
-        AND id BETWEEN #{start_id} AND #{end_id}
-      EOF
-    end
-  end
-
-  def down
-    UserSyncedAttributesMetadata.each_batch do |batch|
-      start_id, end_id = batch.pluck('MIN(id), MAX(id)').first
-
-      execute <<-EOF
-        UPDATE users
-        SET users.email_provider = metadata.provider, users.external_email = metadata.email_synced
-        FROM user_synced_attributes_metadata as metadata, users
-        WHERE metadata.email_synced = TRUE
-        AND metadata.user_id = users.id
-        AND id BETWEEN #{start_id} AND #{end_id}
-      EOF
-    end
-  end
-end