Cleanup non-embedded Stripe KYC code

[oaratovskyi]

Description

Perform code cleanup: remove the code that dealt with the old method of redirecting to external KYC.
We should wait until the embedded KYC feature has been out there for a few months first.
Follow up of #9445

Dev notes

• Old method start here:
  

  woocommerce-payments/includes/class-wc-payments-account.php

  Line 1890 in 7ca58a8

  $return_url = $this->get_onboarding_return_url( $wcpay_connect_from );

• Remove loading step and its test