Remove api user and password for sipgate integration

[MrGeneration]

Infos:

• Used Zammad version: stable / 5.0.2
• Installation method (source, package, ..): any
• Operating system: any
• Database + version: any
• Elasticsearch version: any
• Browser + version: any
• Ticket: #1095687

Expected behavior:

Zammad uses all required settings.

Actual behavior:

Zammad requests a api user and password for the sipgate integration.
However, technically they're not required and code wise not used.

We thus should remove this to reduce confusion and complexity.

Steps to reproduce the behavior:

• have a look at the sipgate integration

Yes I'm sure this is a bug and no feature request or a general question.