Migrate away from adding FMassRequestEnterVehicleFragment on entities

[Leroy231]

For better performance. Instead:

• [x] Add a var to FMassVehicleOccupantFragment FMassEntityHandle RequestedVehicleToEnter
• [x] Everywhere we check for existence of FMassRequestEnterVehicleFragment instead check FMassVehicleOccupantFragment.RequestedVehicleToEnter.IsSet()
• [x] Everywhere we check FMassRequestEnterVehicleFragment.VehicleToEnter instead check FMassVehicleOccupantFragment.RequestedVehicleToEnter
• [x] Wherever we add FMassRequestEnterVehicleFragment or update FMassRequestEnterVehicleFragment.VehicleToEnter instead update FMassVehicleOccupantFragment.RequestedVehicleToEnter
• [x] Where we delete FMassRequestEnterVehicleFragment instead call FMassVehicleOccupantFragment.RequestedVehicleToEnter.Reset()
• [x] Delete FMassRequestEnterVehicleFragment