Remove sql.Base.get_session

With the change to use oslo's db.sqlalchemy.sessions, the
sql.Base class isn't doing enough to justify its continued
existence.

This change is removing the get_session member, which was just
a new name for openstack.common.db.sqlalchemy.session.get_session() .

Part of bp use-common-oslo-db-code

Change-Id: I083ecd54200dcd6cae7248d1a6ef4fcec64e6589
Merge "Remove sql.Base.get_session"