There is no "delete" option for aircraft since reservations are associated with that aircraft. Therefore, if you were to delete it, it will permanently delete all past reservation details for that aircraft.
Instead, simply click on the "edit" link beside the aircraft and change the Visibility option to "Always Hidden." This will make it so the aircraft can no longer be reserved and will no longer be visible to your members. In addition, that will open an aircraft slot if you purchase a new asset later on.