Remove references to username/password

[michel-laterman]

What does this PR do?

Remove the use of username/password for authenication

Why is it important?

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Create an 8.1 cluster and use a snapshot of the fleet-server from elastic/fleet-server#1006

Related issues

• Relates fleet-server username/password authentication must be removed in 8.0 fleet-server#978

[elasticmachine]

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Build stats

• Start Time: 2021-12-22T20:38:29.844+0000

• Duration: 99 min 29 sec

• Commit: 47181e3

Test stats 🧪

Test	Results
Failed	0
Passed	7120
Skipped	16
Total	7136

💚 Flaky test report

Tests succeeded.

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