Is it necessary to discharge the capacitors on an AC condensor?
Most of the motors associated with fans and such, utilize an induced magnetic field, and don't have a permanent magnet. So as the capacitor releases its charge (quickly), a magnetic field would be induced in the motor and cause the motor to turn maybe once or twice, and that should spend most, if not all, of the the stored charge. So the capacitor will release the charge to turn the motor... or more technically, the motor will draw the stored current out of the capacitor (quickly).

With respect to the kind of motor that is turning, for most any AC fan motor, it is highly doubtful that a dynamo-style (using a permanent magnet) motor is being used (except on wind mills). I mention this because the efficiency of the discharging effect could be different compared with a dynamo-style motor... but I also think it should be kind of the same (roughly). The difference being that inertia of the spinning dynamo, will continue to generate electricity, until it nearly stops- at which point it would draw the stored current from the capacitor.

But something else to be aware of is that, typically, large capacitors do not perfectly insulate or isolate or hold charges... they naturally discharge within themselves after a while. Furthermore, large capacitors usually have a bleeder resistor attached to the capacitors terminals. Discharging through motors and other electrical items where a circuit exists is (probably) the primary way, though.