[connection] remove ensure_cnx_set context manager uses
It has been deprecated in a previous changeset and is now a no-op.
Related to #2919309.
for table, column in [
('transactions', 'tx_time'),
('tx_entity_actions', 'tx_uuid'),
('tx_relation_actions', 'tx_uuid')]:
repo.system_source.create_index(session, table, column)
commit()