Use this code in Appearance > Customize > Additional CSS
.archive.category .main .page-title { position: relative; margin-left: -3.15em; }
.archive.category .main .page-title:before { content: "Category: "; position: absolute; color: #fff; background: #fff ; display: inline-block; }
It works in both small (mobile) screen and desktop screen. I tested it with theme demo’s category archive page using Chrome web dev tool. But it might not work when font family differs from the one used in demo, if that’s the case try adjusting -3.15em
until it’s hidden completely.
Yes, it is a hackish CSS way to hide that part but it gets the job done and no worry of breaking php template or function.