pubsub admin clients contain references to data plane operations in service description

[suzmue]

After removing the data plane operations from the admin clients, we still have references to the data plane operations that do not exist. We may want to replace these with custom descriptions or add a warning.

For example the Subscription client:

See issues #3010 and #3011

[coryan]

I think a description-override in the [source] section can do the trick.