Remove unused columns from table fahrplan

[martinheppner]

• [ ] connection_returns_min_waiting_duration
• [ ] connection_returns_max_waiting_duration
• [ ] connection_returns_warning_level
• [ ] connection_returns_warning
• [ ] return_waiting_duration
• [ ] connection_lastregular_arrival_datetime

[martinheppner]

@diditrummer I would like to remove this columns. What is your point of view?