Remove obsolete files in `<admin\extension>` and `<catalog\extension>` subfolders

[gizmecano]

Remove obsolete files in <admin\extension> and <catalog\extension> subfolders (minor)

1. <admin\extension\module>

   • [x] pp_button.php
   • [x] pp_login.php

2. <admin\extension\payment>

   • [x] pp_express.php
   • [x] pp_express_order.php
   • [x] pp_express_refund.php
   • [x] pp_express_search.php
   • [x] pp_express_view.php
   • [x] pp_payflow.php
   • [x] pp_payflow_iframe.php
   • [x] pp_pro.php
   • [x] pp_pro_iframe.php
   • [x] pp_standard.php

3. <catalog\extension\payment>

   • [x] pp_express.php
   • [x] pp_payflow.php
   • [x] pp_payflow_iframe.php
   • [x] pp_pro.php
   • [x] pp_pro_iframe.php
   • [x] pp_standard.php