Remove unused argument --target_platform in Python Tool

[parthosa]

Fixes #1277. This PR removes the usused argument --target_platform. It is used to recommend cluster shape and cost savings for onprem platform.

        :param target_platform: Cost savings and speedup recommendation for comparable cluster in
                target_platform based on on-premises cluster configuration.

                Currently only `dataproc` is supported for target_platform.
                If not provided, the final report will be limited to GPU speedups only without
                cost-savings.

This argument should be removed because:

1. For onprem, Scala Tool now generates recommended cluster shape and tuning configs
2. We do not do cost savings.