fix: remove superfluous line of code in solution (#528)

Author: Brian Lister

animation/02-pop-up/solution/solution.js

@@ -1,6 +1,5 @@
 const openButton = document.getElementById('trigger-modal');
 const closeButton = document.getElementById('close-modal');
-const backdrop = document.getElementById('backdrop')
 
 function toggleModal() {
   const modalDiv = document.querySelector('.popup-modal');