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If you are modifying native post types (post or page), you can do so in your file functions.php
add_action( 'init', 'retira_editor' );
function retira_editor() {
remove_post_type_support( 'post', 'editor' );
remove_post_type_support( 'page', 'editor' );
}
ref: remove_post_type_support()
If it is a Custom Post Type that you created, at the time of creating the CPT you can pass like this:
$args = array(
'labels' => $labels,
/* omiti os outros parâmetros */
'supports' => array( 'title' ),
);
register_post_type( 'nome', $args );
Thus, the post of type "name" will have only the title.
ref: register_post_type()