remove unused reference

Author: LaviniaStiliadou

angular.json

@@ -22,7 +22,6 @@
             ],
             "styles": [
               "src/styles.scss",
-              "src/app/css/app.css",
               "node_modules/bpmn-js/dist/assets/diagram-js.css",
               "node_modules/bpmn-js/dist/assets/bpmn-font/css/bpmn-embedded.css",
               "node_modules/bpmn-js-properties-panel/styles/properties.less"