DATAMONGO-2068 - Avoid registration of a default MongoMappingContext bean.
The default registration causes a bean of type MongoMappingContext to always be available in the ApplicationContext which makes Spring Boot's auto-configuration that applies entities obtained by a custom scan to the context. This will cause our default one MappingContext not see those entities during initialization and thus polymorphic queries to not work properly as type aliases require the entities to having been inspected already.
In manual configuration, users will have to take care of the proper bean declarations already or just extend AbstractMongoConfiguration which will ensure the proper setup anyway.
Related tickets: spring-projects/spring-boot#14177
