Remove obsolete metadata

To get rid of Hasura console warning.
The actual constraints were removed already in daee0fa

metadata/hsl/databases/timetables/tables/return_value_timetable_version.yaml

@@ -1,13 +1,3 @@
 table:
   name: timetable_version
   schema: return_value
-object_relationships:
-  - name: day_type
-    using:
-      foreign_key_constraint_on: day_type_id
-  - name: vehicle_schedule_frame
-    using:
-      foreign_key_constraint_on: vehicle_schedule_frame_id
-  - name: substitute_operating_day_by_line_type
-    using:
-      foreign_key_constraint_on: substitute_operating_day_by_line_type_id