When security group is deleted, log entries configured for that SG
weren't removed and stayed in the DB as orphaned entries.
That patch fixes that by subscribe logapi service plugin to the
AFTER_DELETE security group events.
Conflicts:
neutron/tests/unit/services/logapi/test_logging_plugin.py
Closes-bug: #1939558
Change-Id: Ibffbbd3b529c31372715d9a064f7cb6bada8c424
(cherry picked from commit 7f06322355)
(cherry picked from commit 9d6aaff9a1)