Remove the word ‘Categories’ from wp_list_categories

Take a look at the parameters in the documentation of wp_list_categories() function.

You need to use this parameter:

title_li

(string) Set the title and style of the outer list item. Defaults to “Categories”. If present but empty, the outer list item will not be displayed. See below for examples.

I think it’s clear, you just need to pass this parameter as an empty string:

wp_list_categories('title_li=');