Remove OXID class from metadata.php

[zckman]

This removes the use of the OXID Registry since it is not actually used and problematic for some OXID 6.2 setups if OXID classes are not available during installation.

• metadata.php should not contain additional source code since 6.2: https://docs.oxid-esales.com/eshop/en/6.2/releases/releases-2020/oxid-eshop-620.html#changes-in-the-module-system
• New Metadata version 2.1: "No other variables or code are allowed." https://docs.oxid-esales.com/developer/en/6.2/development/modules_components_themes/module/skeleton/metadataphp/version21.html