MAGETWO-90375: Removing "Rush" theme work from 2.3.0

- removing rush theme from composer

Author: cpartica

composer.json

@@ -232,7 +232,6 @@
         "magento/theme-adminhtml-backend": "*",
         "magento/theme-frontend-blank": "*",
         "magento/theme-frontend-luma": "*",
-        "magento/theme-frontend-rush": "*",
         "magento/language-de_de": "*",
         "magento/language-en_us": "*",
         "magento/language-es_es": "*",