Most of the electric models don't need the RX battery pack. The modern electronic speed controls have a motor power cut-off that kills the motor while you've still got enough battery to bring the plane down safely.

Read the instruction manuals for your ESC, and for your model. You'll probably find that the AAA pack can be left out with no problem at all. If this was a package deal, the RX battery holder was probably included just so you'd have it in case you used the radio in a plane that wasn't using the electric drive ystem.