Remove use of spark.worker.instances

spark.worker.instances was added as part of this commit: https://github.com/apache/spark/commit/1617816090e7b20124a512a43860a21232ebf511

My understanding is that SPARK_WORKER_INSTANCES is supported for backwards compatibility,
but spark.worker.instances is never used (SparkSubmit.scala sets spark.executor.instances) so should
not have been added.

@sryza @pwendell @tgravescs LMK if I'm understanding this correctly