This reverts commit 062336e59bb3d427bf4094779dc8e93d92c078d5.
Now, we have proper fix for the system_scope='all' in elevated context
in the neutron-lib so we can revert temporary fix made at the end of the
Wallaby cycle.
Related-Bug: #1920001
Conflicts:
neutron/api/rpc/agentnotifiers/dhcp_rpc_agent_api.py
neutron/common/utils.py
neutron/db/address_group_db.py
neutron/services/segments/db.py
Change-Id: Ife9b647b403bdd76a8a99984ea8858bf95c96bc3