MikeJones Yeah, it's a bit different now, the exact replacement for your code would be as follows:
a[data-fancybox="responsive"]:hover {
border-bottom: none!important;
}
For people who are interested, in case you don't want the underline to appear ever, with/without hover on the image, it would be like this:
a[data-fancybox="responsive"] {
border-bottom: none!important;
}