What version of X3 are you using? Since release X3 0.22, the popup topbar will auto-hide after a few seconds, so I am not quite sure why you want it removed ... If you remove it, depending on the aspect of the image vs screen, the icons in top right corner may be invisible to the visitor.
If you want to proceed, add the following to
settings -> custom -> custom CSS:
.pswp__top-bar {
background-color: rgba(0,0,0,0) !important;
}
Or if you want to always hide the topbar, including the buttons:
.pswp__top-bar {
display: none !important;
}