Remove container aware migrations documentation

It's not recommended to use services in migrations because they could change or be removed, which could leave your database in an unpredictable state.

See #243 for more information.

Resources/doc/index.rst

@@ -329,67 +329,6 @@ If you don't want to use this workflow and instead create your schema via
 
 Otherwise Doctrine will try to run all migrations, which probably will not work.
 
-Container Aware Migrations
---------------------------
-
-In some cases you might need access to the container to ensure the proper update of
-your data structure. This could be necessary to update relations with some specific
-logic or to create new entities.
-
-Therefore you can just implement the ContainerAwareInterface with its needed methods
-to get full access to the container or ContainerAwareTrait if you use Symfony >= 2.4.
-
-.. code-block:: php
-
-    // ...
-    use Symfony\Component\DependencyInjection\ContainerAwareInterface;
-    use Symfony\Component\DependencyInjection\ContainerInterface;
-
-    class Version20130326212938 extends AbstractMigration implements ContainerAwareInterface
-    {
-        private $container;
-
-        public function setContainer(ContainerInterface $container = null)
-        {
-            $this->container = $container;
-        }
-
-        public function up(Schema $schema)
-        {
-            // ... migration content
-        }
-
-        public function postUp(Schema $schema)
-        {
-            $converter = $this->container->get('my_service.convert_data_to');
-            // ... convert the data from markdown to html for instance
-        }
-    }
-
-With the trait
-
-.. code-block:: php
-
-    // ...
-    use Symfony\Component\DependencyInjection\ContainerAwareInterface;
-    use Symfony\Component\DependencyInjection\ContainerAwareTrait;
-
-    class Version20130326212938 extends AbstractMigration implements ContainerAwareInterface
-    {
-        use ContainerAwareTrait;
-
-        public function up(Schema $schema)
-        {
-            // ... migration content
-        }
-
-        public function postUp(Schema $schema)
-        {
-            $converter = $this->container->get('my_service.convert_data_to');
-            // ... convert the data from markdown to html for instance
-        }
-    }
-
 Manual Tables
 -------------