Remove un-necessary NodePorts from system test.

[see-quick]

In HttpBridgeKafkaExternalListenersST.java (#7526), we have a test case (i.e., testWeirdUsername), which currently uses NodePort. Fortunately, we can remove it, because it is leftover during the refactoring of test clients by Lukas.

IIRC reason why we are using external producer etc. was that at the time I was writing those "weird username tests", there were no possibilities to use internal clients - as they didn't allow any username to be longer than some number of characters and it shouldn't contain any special characters. External clients are allowing it.

Test cases currently do the following:

1. Create a bridge consumer to receive messages from the external listener (NodePort)
2. Create a producer to produce messages via the same listener (NodePort)

But we can easily modify 2. point to use internal listeners instead of NodePort.