Remove duplicated message (#7360)

i18n/en.json

@@ -407,10 +407,6 @@
     "id": "app.plugin.disabled.app_error",
     "translation": "Plugins have been disabled by the system admin or the server has not been restarted since they were enabled."
   },
-  {
-    "id": "app.plugin.disabled.app_error",
-    "translation": ""
-  },
   {
     "id": "app.plugin.extract.app_error",
     "translation": "Encountered error extracting plugin"