Remove obsolete v1 flags, cloudbuild steps, and related configurations

[stephen-soltesz]

This change continues the decommission of the v1 logic by removing obsolete flags from etl_worker.go, removing commented steps from cloudbuild.yaml, and removing obsolete Dockerfile for the worker.

This change also updates the -output_dir flag to -output_location so that it can be used for both output types, "local" or "gcs". This makes the output bucket configurable, where before it was unconditionally etl-<PROJECT>.

Part of:

• https://github.com/m-lab/etl/issues/1084

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 7356

• 2 of 3 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 67.17%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cmd/etl_worker/etl_worker.go	2	3	66.67%
<!--	Total:	2	3	66.67%	-->

Totals
Change from base Build 7352:	0.05%
Covered Lines:	3335
Relevant Lines:	4965

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💛 - Coveralls