Delete post meta conditionally after save post

• Well, first use the hook properly. The post ID will be passed in. You
  don’t need $post->ID.
• Second, use the correct hook. If you want to run save_post only for
  your booking type, use save_post_booking

But otherwise, the code works. I just ran a quick test.

function booking_status_is_updated($post_id){
  if(get_post_meta($post_id,'booking_status',true)=='denied'){
    delete_post_meta($post_id,'booking_slot');
  }
}
add_action('save_post_booking','booking_status_is_updated');