If you want to remove "Category Archives:" and keep the Category's name on Wordpress site follow the steps below:
Install and activate RTFR.
The Real-Time Find and Replace plugin will be under TOOLS menu
1. Click "Add"
2. Replace (left box)
>Category Archives: <
with (right box)
3. Finally Click "Update Settings." The Category Archive will be gone from your WordPress site.
You might also want to replace (using the same technique):