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

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

• 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

To be clarified

Currently virtual timeslots are not fully implemented in Engine. Compare comments and related MRs.

Let's discuss, how we are dealing with the missing virtual timeslots parts. Either we fix it as part of this ticket or we get rid of the engine-api virtual timeslot requirement, by implementing aura#312 and finalize other virtual timeslot handling details later.

Dependencies

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