Drop pymongo useless and removed attribute

The current version of oslo.cache isn't compatible with the recent
major upgrade of pymongo [1].

These changes remove the usage of this attribut as it is useless
and removed from the recent versions of pymongo [2]. That will
fix our gates.

[1] https://opendev.org/openstack/requirements/commit/7ea81016529904ba3c1d593fea4ee05e5a66e5ad
[2] https://pymongo.readthedocs.io/en/stable/migrate-to-pymongo4.html?highlight=MongoReplicaSetClient#mongoreplicasetclient

Change-Id: I21ad7023b14ff2ad33ddd036bfecc905d2c68871