Take an angle grinder and cut a small hole in the bottom of the tank - don't worry, once there is only fumes left in the tank, you can use a stick welder to close the hole.
*disclaimer, probably don't do this, but if you do take a video at least.
why though? did you put in petrol into a diesel for example? draining fuel out through the fuel pump is not always advised depending on the reason and also wont get everything out.