Remove Unused CSS From Prometheus Settings

The margin left is now applied via a utility class, and also is no
longer a success button, so this can go.

app/assets/stylesheets/pages/settings.scss

@@ -278,10 +278,6 @@
     .card-header {
       display: flex;
       align-items: center;
-
-      > .btn-success {
-        margin-left: auto;
-      }
     }
 
     .custom-metric {