title: Avoid billing phone validation in WooCommerce checkout
body: <p>I have a problem with WooCommerce billing phone validation.
  I installed YITH checkout manager for edit checkout fields and delete a phone number field, but when I try going to the next step I see the error for phone validation! Also I added below code in my functions, but there was no change.
  help me to fix this error.</p>
  
  <pre><code>add_filter('woocommerce_checkout_fields', 'no_phone_validation')
  
  function no_phone_validation($woo_checkout_fields_array)
  {
      unset($woo_checkout_fields_array['billing']['billing_phone']['validate']);
      return $woo_checkout_fields_array;
  }
  </code></pre>