[request] drop the user argument for set_session
I see no code nor test that use this optional argument. removing it help to
clean the session code. The set_session function will soon be deprecated anyway (at the
same time than the dbapi)
related to #2503918
from yams.buildobjs import RelationDefinition
class ARelation(RelationDefinition):
subject = 'CWUser'
object = 'CWGroup'