css: Optimize performance

I don't quite understand why exactly this rule exposes
such an issue. We have several other rules that are
similar. But they don't reference form elements on the
left. I suspect a different issue somewhere else, this
only exaggerated it.

fixes #4929

public/css/icinga/forms.less

@@ -560,7 +560,8 @@ form.icinga-form .form-info {
     box-shadow: 0 0 .25em 0 rgba(0,0,0,.4);
   }
 
-  &[disabled] ~ * {
+  &[disabled] ~ img,
+  &[disabled] ~ span {
     opacity: .25;
   }