Remove polyfill-util dependancy from security-core

Security-core no longer directly depends upon polyfill-util since #16382.

This does not change the existing dependancy tree as polyfill-util is
transitivly depended on via polyfill-php56.

src/Symfony/Component/Security/Core/composer.json

@@ -17,8 +17,7 @@
     ],
     "require": {
         "php": ">=5.5.9",
-        "symfony/polyfill-php56": "~1.0",
-        "symfony/polyfill-util": "~1.0"
+        "symfony/polyfill-php56": "~1.0"
     },
     "require-dev": {
         "symfony/event-dispatcher": "~2.8|~3.0",