delete unnecessary class

src/client/styles/scss/_admin.scss

@@ -137,10 +137,6 @@ $slack-work-space-name-card-border: #efc1f6;
       background-color: $slack-work-space-name-card-background;
       border: 1px solid $slack-work-space-name-card-border;
     }
-    .growi-bot-kun {
-      width: 20px;
-      height: 20px;
-    }
   }
 
   //// TODO: migrate to Bootstrap 4