Virtual Timeslots: Remove workarounds and replace with Steering's native approach

Parent: [EPIC] Display virtual timeslots / Music Fallba... (aura#176)

• Remove the config for station fallback show and current workaround to create virtual timeslots
• Compare the final implementation in Steering: steering#154 (closed)
• After Steering /playout endpoint has a valid schema, also update the schema in engine/schemas/openapi-steering.json

Dependencies

• API: Update API client, after Steering provides... (#135)