Remove the workaround used in EntityManagerTest

[sai-pullabhotla]

A couple of tests in EntityManagerTest were workedaround for the below issue. The latest version of DatastoreEmulator has fixed this issue, so we can remove the workaround.

DatastoreException - Error Code/Reason are inconsistent when working with Emulator #2442

[sai-pullabhotla]

Need to do this in DatastoreTransactionTest and DatastoreBatchTest as well.

[sai-pullabhotla]

Found that the builds are failing on Travis as Travis Builds use LocalDatastoreHelper for downloading and running the Emulator. The LocalDatastoreHelper is still downloading version 1.2.0 of the Emulator which still has this issue. Reported this to the google-cloud team. May have to revert the changes, unless the LocalDatastoreHelper is fixed soon.