[0/4] Remove user code from ExecutionPlan steps
Summary: This is the part of https://dagster.phacility.com/D6133 that doesn't deal with plan snapshot serialization and rebuilding the ExecutionPlan from the snapshot. Just removes the definition classes from the data objects that make up the ExecutionPlan.
Test Plan: Integration
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D6155