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Thanks for your answer Christophe!
Please some more help. I am trying to implement the suggestions.
But I cannot find the file custom.css. Not in mytheme/assets.
And not in mytheme/assets/css. There are only theme.css and error.css 

So I want to try the second suggestion.
But what is meant by: "override"? Should I put a php-command there?

Edit: I think I found it.
I commented out block subcategory_list and now the subcategory menu is gone.