Update payment gateways docs to remove unsupported/D7 modules

[pameeela]

While reviewing https://docs.drupalcommerce.org/v2/developer-guide/payments/gateways/#135-additional-payment-gateways I noticed that a number of these are no longer supported, mostly because they never had a stable release or don't have any releases for current versions of Drupal.

I can imagine it is a difficult task to keep this up to date, but removing the outdated modules would be a great help! Here are some for example with no current release:
https://www.drupal.org/project/commerce_atom_payment
https://www.drupal.org/project/commerce_atos_sips
https://www.drupal.org/project/commerce_bambora
https://www.drupal.org/project/commerce_credomatic
https://www.drupal.org/project/commerce_payway