"sync" parameter in "PipelineJob.run" method is unused

The ["sync"](https://github.com/googleapis/python-aiplatform/blob/main/google/cloud/aiplatform/pipeline_jobs.py#L307-L308) parameter in the [`PipelineJob.run`](https://github.com/googleapis/python-aiplatform/blob/main/google/cloud/aiplatform/pipeline_jobs.py#L288) (and `PipelineJob._run`) method(s) is described to determine the blocking behaviour of running a Vertex pipeline.
However, in actuality the parameter is unused and [the pipeline execution is always performed blockingly](https://github.com/googleapis/python-aiplatform/blob/main/google/cloud/aiplatform/pipeline_jobs.py#L351).