Fixes #5533: Remove filter gradient from disabled links on dropdowns

docs/assets/css/bootstrap.css

@@ -2950,6 +2950,7 @@ table th[class*="span"],
   cursor: default;
   background-color: transparent;
   background-image: none;
+  filter: progid:DXImageTransform.Microsoft.gradient(enabled=false);
 }
 
 .open {

less/dropdowns.less

@@ -115,6 +115,7 @@
   text-decoration: none;
   background-color: transparent;
   background-image: none; // Remove CSS gradient
+  .reset-filter();
   cursor: default;
 }