Change and simplify algorithm for vehicle repair cost.

This is due to absurdly high repair costs for old vehicles with
old algorithm, due to implicit exponential increases.
Get rid of variable in vehicle struct.

src/vehicle_base.h

@@ -242,7 +242,6 @@ public:
 	Money profit_last_year;             ///< Profit last year << 8, low 8 bits are fract
 	Money profit_lifetime;              ///< Profit lifetime << 8, low 8 bits are fract
 	Money value;                        ///< Value of the vehicle
-	Money repair_cost;                  ///< Cost to repair one breakdown
 
 	CargoPayment *cargo_payment;        ///< The cargo payment we're currently in