Deprecate getParentFieldDescription from ModelManagerInterface

[franmomu]

Subject

Taking a look at the method ModelManager::getParentFieldDescription() from SonataDoctrineMongoDBAdminBundle (and the one in SonataDoctrineORMBundle), looks like they are not used anymore, in fact, the first parameter they receive ($parentAssociationMapping) is an array and then there is this code (which would fail because of using an array as a key):

$associatingMapping = $metadata->associationMappings[$parentAssociationMapping];

This method was added in b0df44f, and the $parentAssociationMapping parameter back then was a string. There was this call in the AbstractAdmin class (previously known as Admin):

$fieldDescription = $this->getModelManager()->getParentFieldDescription($this->getParentAssociationMapping(), $this->getClass());

Then in 789802d that call was replaced by calling to the getParentFieldDescription within the Admin class.

There are maybe other uses of this method I haven't found, but looks like this method is not used.

I am targeting this branch, because these changes are BC.

Changelog

### Deprecated
- Deprecated `ModelManagerInterface::getParentFieldDescription`.

[VincentLanglet]

We should make PR on storages bundle to remove this method too.

[jordisala1991]

Thank you @franmomu