Remove the parent name from permalink

Add this code to your functions.php,

add_action( 'init', 'build_taxonomies', 0 );  

function build_taxonomies() {

  register_taxonomy( 'category', 'post', array(
        'hierarchical' => true,
        'update_count_callback' => '_update_post_term_count',
        'query_var' => 'category_name',
        'rewrite' => did_action( 'init' ) ? array(
                    'hierarchical' => false,
                    'slug' => get_option('category_base') ? get_option('category_base') : 'category',
                    'with_front' => false) : false,
        'public' => true,
        'show_ui' => true,
        '_builtin' => true,
    ) );

}