Clarify and Simplify "Authentication"

[AxelNennker]

Problem description
The API definition has this definition in the "Relevant terms and definitions" section.

"Authentication: Security access keys such as OAuth 2.0 client credentials used by client applications to invoke the QoD API."

But there is a whole section "Authorization and authentication".

Having both is confusing and potentially misleading.

Expected action
Remove

"Authentication: Security access keys such as OAuth 2.0 client credentials used by client applications to invoke the QoD API."