fixes #11126: remove box-shadow from open dropdown toggle with .btn-link

dist/css/bootstrap.css

@@ -3716,6 +3716,11 @@ input[type="button"].btn-block {
           box-shadow: inset 0 3px 5px rgba(0, 0, 0, 0.125);
 }
 
+.btn-group.open .dropdown-toggle.btn-link {
+  -webkit-box-shadow: none;
+          box-shadow: none;
+}
+
 .btn .caret {
   margin-left: 0;
 }

less/button-groups.less

@@ -150,6 +150,11 @@
 // Remove the gradient and set the same inset shadow as the :active state
 .btn-group.open .dropdown-toggle {
   .box-shadow(inset 0 3px 5px rgba(0,0,0,.125));
+
+  // Show no shadow for `.btn-link` since it has no other button styles.
+  &.btn-link {
+    .box-shadow(none);
+  }
 }