Bug 1656293 - Enable ESLint no-unused-vars rules on docshell/test/chrome/*.xhtml files. r=nika

Differential Revision: https://phabricator.services.mozilla.com/D100450

.eslintrc.js

@@ -395,7 +395,6 @@ module.exports = {
         "no-sequences": "off",
         "no-shadow": "off",
         "no-undef": "off",
-        "no-unused-vars": "off",
         "no-useless-call": "off",
       },
     },

docshell/test/chrome/bug113934_window.xhtml

@@ -49,9 +49,8 @@
     $("f3").setAttribute("src", doc2);
 
     async function doTheTest() {
-      var s1 = await snapshotWindow($("f1").contentWindow);
       var s2 = await snapshotWindow($("f2").contentWindow);
-      var s3 = await snapshotWindow($("f3").contentWindow);
+      // var s3 = await snapshotWindow($("f3").contentWindow);
 
       // This test is broken - see bug 1090274
       //ok(!compareSnapshots(s2, s3, true)[0],

docshell/test/chrome/test_bug454235.xhtml

@@ -23,7 +23,6 @@ SimpleTest.waitForExplicitFinish();
 SimpleTest.waitForFocus(doTest);
 
 function doTest() {
-  var shownBrowser = document.getElementById("shownBrowser");
   var hiddenBrowser = document.getElementById("hiddenBrowser");
   var offScreenBrowser = document.getElementById("offScreenBrowser");
   var offScreenSubBrowser = offScreenBrowser.contentDocument.getElementById("topBrowser");