When a bridge interface was reconfigured, running containers using
this bridge lost connectivity: restarting network-addresses-brN.service
triggered a restart of network-setup.service via a "partOf" relationship
introduced in 07e0c0e0a2f237639600f2a0d62f6eac748b1e6e.
This in turn restarted brN-netdev.service.
The bridge was thus destroyed and recreated with the same name but a new
interface id, causing attached veth interfaces to lose their connection.
This change removes the "partOf" relationship between
network-setup.service and network-addresses-brN.service for all bridges.
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