Administration: Remove any CSS related to Internet Explorer versions 6 – 10.

In WordPress 3.2 support for IE6 was dropped, IE7 followed a few versions later. With the 4.8 release, WordPress officially ended support for Internet Explorer versions 8, 9, and 10. Yet, we still have shipped CSS for the unsupported IE versions....until now! Goodbye to ie.css and star hacks!

* Removes ie.css and `ie` style handle.
* Removes IE specific class names and any related CSS.
* Drops support for IE8 and older in `wp_customize_support_script()`.
* Updates compatibility mode for CSS minification to `ie11`.

Props ayeshrajans, isabel_brison, afercia, netweb, peterwilsoncc, ocean90.
Fixes #17232, #46015.
Built from https://develop.svn.wordpress.org/trunk@47771


git-svn-id: https://core.svn.wordpress.org/trunk@47547 1a063a9b-81f0-0310-95a4-ce76da25c4cd