This should work:
.category-box-heading .d-icon-lock {
display: none;
}
depending on the settings you have enabled the classes change and therefore also the elements to be targeted will change.
If it does not work and you can not find a solution send me a link to your site (if it’s the one on your user card I can not open it, my ISP blocks it and I have to go and change the DNS on the router)