remove backfill check for step selection
Enable launching of backfill runs with no previous run, steps selected.
A) Steps without inputs required succeed
B) Steps that do require inputs get a runtime error DagsterStepOutputNotFoundError (currently attributed as a framework error)
Test Plan: Manually triggered backfill with no previous run, steps selected
Reviewers: sandyryza, bengotow
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D4812