remove backfill check for step selection

Summary:
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