Remove unused DOM elements

Changes effectively nothing. Just saves a few unnecessary bytes.
One of the removed variables was the DOM element for the retro computer monitor frame (var overlay = document.getElementById("overlay")), which ended up having no Javascript used on it, and the other was for the ASCII art head at the top (var docHead = document.getElementById("head")), which also never had JS used on it.

index.js

@@ -7,8 +7,6 @@ var convert = document.getElementById("convert");
 var shellType = document.getElementById("shell");
 var copy = document.getElementById("copy");
 var copyalert = document.getElementById("copyalert");
-var overlay = document.getElementById("overlay");
-var docHead = document.getElementById("head");
 var hiddenCredit = document.getElementById("hiddenCredit");
 var nlCheck = document.getElementById("nlCheck");
 
@@ -272,4 +270,4 @@ nlCheck.addEventListener("click", function () {
         nlCheck.style.backgroundColor = "black";
         nlCheck.style.textShadow = "none";
     }
-});
+});