Remove a weird white border from Pipelines page in dark mode

by removing unused CSS

Changelog: fixed
Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

app/assets/stylesheets/page_bundles/pipelines.scss

@@ -182,11 +182,6 @@ button.gl-button.btn-link.mini-pipeline-graph-dropdown-toggle {
     border-bottom-color: $border-color;
   }
 
-  &::after {
-    margin-top: 1px;
-    border-bottom-color: $white;
-  }
-
   /**
    * Center dropdown menu in mini graph
    */