Remove old page for Software Development Kits (SDK)

[dsabelni]

Please remove the old page for Software Development Kits (SDK) - https://docs.otc.t-systems.com/developer/sdk.html

This page is no longer accessible via the left menu panel, and was replaced by https://docs.otc.t-systems.com/developer/sdk/index.html. But the original outdated page still can be accessed via direct link and doesn't contain the option to go to Third-Party Software Development Kits (SDK), which potentially can be confusing for the customer (for example, in case the original page was saved as a bookmark in a browser).