Remove Category description textarea

/*remove term descriptions from post editor */

function wpse_hide_cat_descr() { ?>

    <style type="text/css">
       .term-description-wrap {
           display: none;
       }
    </style>

<?php } 

add_action( 'admin_head-term.php', 'wpse_hide_cat_descr' );
add_action( 'admin_head-edit-tags.php', 'wpse_hide_cat_descr' );

If you need to target it to category editor only – in other words leave description for other taxonomies, then easiest will be to position the .taxonomy-category body class before the .term-description-wrap. If for some reason you want to remove only the textarea, it has an ID of #description.

The first action removes the category description box from the category and tags ‘edit’ screens, while the second action removes it from the ‘add new’ screens.