Revert "Properly require database tasks so we have access to the raised constant"

This reverts commit fbcd46b1.

This is not needed anymore. See 72809654

activerecord/test/cases/tasks/mysql_rake_test.rb

 require 'cases/helper'
-require 'active_record/tasks/database_tasks'
 
 module ActiveRecord
   class MysqlDBCreateTest < ActiveRecord::TestCase

activerecord/test/cases/tasks/postgresql_rake_test.rb

 require 'cases/helper'
-require 'active_record/tasks/database_tasks'
 
 module ActiveRecord
   class PostgreSQLDBCreateTest < ActiveRecord::TestCase

activerecord/test/cases/tasks/sqlite_rake_test.rb

 require 'cases/helper'
 require 'pathname'
-require 'active_record/tasks/database_tasks'
 
 module ActiveRecord
   class SqliteDBCreateTest < ActiveRecord::TestCase