The best way of removing default category is to change a parameter is to change the parameter ‘default_category’ in your ‘wp_options’ table.

Set the parameter ‘default_category’ to 0. Note that it won’t change the category displayed in your dashboard in the Writing settings, but it will prevent WordPress to set a default term to your post.

WordPress doesn’t require a term or category to be set by default. It’s just not true 😉

Note that this trick, setting the parameter ‘default_category’ to 0 will let you delete Uncategorized category if it bothers you.