If you needed aileron trim it would be because the aircraft had a residual (non zero) rolling moment - which means that the aircraft would actually be continuously rolling at a rate where the roill damping balanced out the rolling moment asymmetry (ignoring cross coupling terms, such as roll due to yaw etc)
Therefore if you have a stable bank angle, the problem isn't due to a rolling moment offset; rather it's indicative of a non-zero sideforce, which is being balanced by banking slightly to use the gravity vector in the aircraft Y-axis to balance the sideforce.
Therefore, to eliminate the bank you need to zero out the sideforce; that means inducing (or possibly removing) sideslip, and the way to do that is by rudder trim.