Clean up dead code in topic enforcer around listing existing topics.

[shk3]

Topic Enforcer lists existing topics with an optional filter to exclude internal topics from enforcement. It was intended to be a switch to allow including internal topics in the results. However, we have never included internal topics for the enforcer, and the current listing code does not work for that logic either -- even if we include internal topics in the Kafka admin client call, the internal topics are still filtered out at the step when we returns.

Given we are not using this include-internal logic, let's just remove it and clean up.