Is an electromagnetic pulse sufficient to account for the complete dscharge of an aircraft struck by lightning?
It is not quite clear what you mean by an electromagnetic pulse and why you seem to assume that an aircraft struck by a lightning will be completely discharged.
So, I'll try to describe what I believe might happen during a strike and, hopefully, it'll clarify matters for you.
As clouds get charged (due to processes outside the scope of this question), they are "looking" for an opportunity to get rid of their charge by sharing it with any conductors close enough to be reached, like earth or other clouds or an opposite side of the same cloud that happen to carry a charge of an opposite sign.
From this prospective, a passing aircraft is a legitimate target, just like another cloud but even better considering that its skin is highly conductive and its geometry includes a lot of sharp edges: both of these features will facilitate the formation of strong electrical fields and the initiation of a discharge.
When a discharge or a lighting strike between a cloud and an aircraft occurs, the aircraft ends carrying a fraction of the charge lost by the cloud, i.e., it will be charged - not discharged.
It is likely that an aircraft was carrying some charge before the strike had occurred.
If so, such charge could make the strike more or less likely, depending on the sign of that charge and the sign of the charge in the cloud, but, once the strike occurs, the end result won't depend on the magnitude of that initial charge - it'll rather be dependent on the magnitude of charge in the cloud and on the relative effective capacitance of the cloud and the aircraft.
This is because the initial charge on the plane is likely to be insignificant with the charge in the cloud and will be, so to speak, lost in transaction.
Even in the absence of charged clouds, a charged aircraft will gain and lose charge continuously as it goes. I suppose that, when it passes through a neutral cloud, it could discharge completely, but I am not sure if such discharge could be violent enough to qualify as a lightning strike.