Remove unused mapping services

[franmomu]

Follow-up of #793

While removing annotation mapping support in #793, I've notice that these services are not necessary.

So mapping drivers are registered in AbstractDoctrineExtension::registerMappingDrivers().

For example for attribute it executes this code which in our case uses doctrine_mongodb.odm.metadata.attribute.class to create the AttributeDriver service.

[GromNaN]

This 2 parameters are no longer necessary.

Suggested change

	<parameter key="doctrine_mongodb.odm.metadata.driver_chain.class">Doctrine\Persistence\Mapping\Driver\MappingDriverChain</parameter>
	<parameter key="doctrine_mongodb.odm.metadata.attribute.class">Doctrine\ODM\MongoDB\Mapping\Driver\AttributeDriver</parameter>

You should also remove the assertions on this parameters in the tests.

[franmomu]

They are still necessary, the thing about how it works is that (for $driverType = 'attribute' for example) this code:

$mappingDriverDef = new Definition($this->getMetadataDriverClass($driverType), [
    array_values($driverPaths),
]);

calls to our:

protected function getMetadataDriverClass(string $driverType): string
{
    return '%' . $this->getObjectManagerElementName('metadata.' . $driverType . '.class') . '%';
}

// ...

protected function getObjectManagerElementName(string $name): string
{
    return 'doctrine_mongodb.odm.' . $name;
}

so it looks for %doctrine_mongodb.odm.metadata.attribute.class%

[GromNaN]

Ok, got it.