[memoization improvements 1/n] get rid of step_keys_to_execute parameter on create_run_for_pipeline, simplify implementation of create_run_for_pipeline.

Summary: We never actually use the step_keys_to_execute_paramater on create_run_for_pipeline in any significant way, and removing it allows for singificant code simplification.

Test Plan: switched around unit tests where necessary.

Reviewers: sandyryza, alangenfeld

Reviewed By: alangenfeld

Differential Revision: https://dagster.phacility.com/D8920