Remove executePlan dagster-graphql call
Summary: The only remaining callsites of this were executing a plan in-process and didn't need to be using dagster-graphql in the first place. Switch those callsites over to call execute_plan directly and then remove executePlan altogether from dagster-graphql.
Test Plan: BK
Reviewers: alangenfeld, johann
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D4817