Remove HTML5 Shiv credit from readme.txt

Author: sabernhardt

src/wp-content/themes/twentythirteen/readme.txt

@@ -38,10 +38,6 @@ GNU General Public License for more details.
 
 Twenty Thirteen Theme bundles the following third-party resources:
 
-HTML5 Shiv v3.7.0, Copyright 2014 Alexander Farkas
-Licenses: MIT/GPL2
-Source: https://github.com/aFarkas/html5shiv
-
 Genericons icon font, Copyright 2013-2017 Automattic Inc.
 License: GNU GPL, Version 2 (or later)
 Source: http://www.genericons.com