KafkaIO's EOS mode does not work with FlinkRunner

KafkaIO has a validation check to allow only certain runners capable of provide exactly-once semantics:

        if ("org.apache.beam.runners.direct.DirectRunner".equals(runner)
            || runner.startsWith("org.apache.beam.runners.dataflow.")
            || runner.startsWith("org.apache.beam.runners.spark.") {
...

The Flink supports exactly-once checkpointing but the Flink Runner can't utilize it in the way KafkaIO intends it.

I think we should remove the check in favor of checking for translation of @RequiresStableInput. Changes to KafkaIO might have to be made to support EOS efficiently with the Flink Runner.