Remove default WYSIWYG editor without removing custom fields editors

WordPress has the remove_post_type_support function that can be used with this purpouse, you can learn more from https://developer.wordpress.org/reference/functions/remove_post_type_support/

Just an example borrowed from codex

/**
 * Remove excerpt support from posts.
 */
function wprocs_custom_init() {
    remove_post_type_support( 'post', 'excerpt' );
}
add_action( 'init', 'wpdocs_custom_init' );