QmlDesigner: Remove unused file

Change-Id: Ife567cfb368092f818c7cf418ede4ab1bca227aa
Reviewed-by: Tim Jenssen <tim.jenssen@theqtcompany.com>

src/plugins/qmldesigner/qmldesignerextension/QmlDesignerExtension.json.in

-{
-    \"Name\" : \"QmlDesignerExtension\",
-    \"Version\" : \"$$QTCREATOR_VERSION\",
-    \"CompatVersion\" : \"$$QTCREATOR_COMPAT_VERSION\",
-    \"Vendor\" : \"The Qt Company Ltd\",
-    \"Copyright\" : \"(C) 2015 The Qt Company Ltd\",
-    \"License\" : [ \"Commercial Usage\",
-                  \"\",
-                  \"Licensees holding valid Qt Commercial licenses may use this plugin in accordance with the Qt Commercial License Agreement provided with the Software or, alternatively, in accordance with the terms contained in a written agreement between you and The Qt Company.\"
-    ],
-    \"Category\" : \"Qt Quick\",
-    \"Description\" : \"Extensions for the Qt Quick Designer.\",
-    \"Url\" : \"http://www.qt.io\",
-    $$dependencyList
-}