Move ee/app/assets/javascripts/prometheus_metrics files

Move ee/app/assets/javascripts/prometheus_metrics files into Core since
the Custom metrics feature is being moved into Core.

app/assets/javascripts/prometheus_metrics/constants.js

@@ -2,4 +2,5 @@ export default {
   EMPTY: 'empty',
   LOADING: 'loading',
   LIST: 'list',
+  NO_INTEGRATION: 'no-integration',
 };

ee/app/assets/javascripts/prometheus_metrics/constants.js

-import PANEL_STATE from '~/prometheus_metrics/constants';
-
-const PANEL_STATE_EE = {
-  NO_INTEGRATION: 'no-integration',
-};
-
-export default Object.assign({}, PANEL_STATE, PANEL_STATE_EE);