Network: Work around OVN lb-add bug that allows multiple Load_Balancer records to be created with same name

[tomponline]

The OVN lb-add command that adds entries to the Load_Balancer OVN northbound table has a bug that in certain situations allows multiple records with the same name to be created.

This then causes issues with the lb-del command that does not support the scenario of duplicate records of the same name.

This fix has multiple facets:

1. Changes the way LXD deletes existing load balancer entries by searching for them by name and then deleting them by UUID, this supports multiple records of the same name existing.
2. Changes the way LXD associates load balancer entries to a logical router by using the add database command directly rather than lr-lb-add that allows multiple records to exist with the same name.
3. Drops unused associated of load balancers to logical switches.

Fixes https://github.com/canonical/lxd/issues/13462