If you don't create a child theme, or at the very least use a Custom CSS plugin, you will lose any customisations when/if the theme is updated.
Onto your question, you want to remove the entire form?
If so, you can disable comments globally in Setting -> Discussion -> uncheck 'Allow people to post comments on new articles'. But if you wanted to make doubly sure your comment form will never show, pop this into your custom CSS:
However, this will also work: