HTML validation: No longer relax the `X-UA-Compatible` error

The validator no longer complains about it unless the value is different from `content=IE=edge`, see https://www.w3.org/Bugs/Public/show_bug.cgi?id=27091

Gruntfile.js

@@ -321,7 +321,6 @@ module.exports = function (grunt) {
         failHard: true,
         reset: true,
         relaxerror: [
-          'Bad value X-UA-Compatible for attribute http-equiv on element meta.',
           'Element img is missing required attribute src.',
           'Attribute autocomplete not allowed on element input at this point.',
           'Attribute autocomplete not allowed on element button at this point.'