Always include the X-UA-Compatible header in `_wp_admin_html_begin()`.

[peterwilsoncc]

_wp_admin_html_begin() uses the following code to avoid compatibility mode in Internet Explorer.

<?php
global $is_IE;
if ( $is_IE ) {
        @header( 'X-UA-Compatible: IE=edge' );
}

As the tag is benign in most cases, the condition can be removed and the HTTP header always included.

In wp-includes/vars.php the compatibility header is set to use chrome frame in the admin (and on the front end if a developer desires).

In the dashboard, these can combined to become X-UA-Compatible: IE=edge, chrome=1.

Once IE11 use drops further, they can be deleted but until then it's a nice trip down memory lane.

[valentinbora]

Related: #49126

[peterwilsoncc]

Closing: WP Admin no longer supports IE 10 or below. IE 11 runs in edge mode by default, so the header is no longer needed.