Remove workaround to run unbounded sources in portable pipelines

In the course of BEAM-6730 we added a way to Runner-translate Reads in portable pipelines. GreedyPipelineFuser was manipulated to let through Reads to the Runner. The better solution for this problem would have been to remove the environment from unbounded reads, which will make it a Runner translated transform.

If in the future we support unbounded reads via a SDF adapter, we can re-add the environment.