Drop usage of JpaAttachments#TRANSACTION_MANAGER and #TSR

Since the WildFly Transaction Client is responsible for coordinating between WildFly and the backing TM implementation, the attachments JpaAttachments#TRANSACTION_MANAGER and JpaAttachments#TRANSACTION_SYNCHRONIZATION_REGISTRY are no longer needed and all usages can be replaced with simple calls to ContextTransactionManager.getInstance() and ContextTransactionSynchronizationRegistry.getInstance().

Note that deployment dependencies (such as those established in PersistenceUnitParseProcessor#deploy) are presently still required (to ensure that the TM is available before starting the deployment), but the value can be discarded.