[#50848327] Remove execution database/data source from work flows if Data Source has been deleted from Chorus

- add workfiles_as_execution_location relations with 'dependent => nullify' to GpdbDatabase and HdfsDataSource
- Renamed workfiles_as_execution_schema to workfiles_as_execution_location for clarity
- workfile_as_execution_location relation in GpdbSchema now uses ':as => :execution_location' to prevent nullifying all execution_location_ids that happen to match the deleted schema's id