Skip authentication for fetching mirror urls

[ugrave]

The self url of the mirror contains already the required authentication credentials.
Therefore its not required to configure the authentication for the request to the mirror

Closes #829

Your checklist for this pull request

• Make sure you are requesting to pull a topic/feature/bugfix branch (right side) and not your master branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub Getting clone url for mirror returns 401 #829
• Link to relevant pull requests, esp. upstream and downstream changes Support cloning from Bitbucket mirror #796
• Did you provide a test-case? That demonstrates feature works or fixes the issue.

[ugrave]

Any update on this?