Remove unnecessary declaration of JtaManager in HibernatePersistenceProviderAdaptor

In the HibernatePersistenceProviderAdaptor, there is an unnecessary declaration of JtaManager. The sole purpose of having this variable is only to use in the overridden method named InjectJtaManager.

This could be improved by removing the JtaManager as there is no use of the variable, as of now at least.