Remove redundant test assertion

[MarkEWaite]

Remove redundant test assertion

Existing assertions already check that the credential usage tracking is correct. This assertion adds no real value to the test.

Part of the fix for issue:

• HTML id for 'usage' has been removed from individual credential pages credentials-plugin#1032

Detected in plugin BOM pull request:

• Update dependency org.jenkins-ci.main:jenkins-war to v2.556 bom#6504

Testing done

• Confirmed that tests pass with older credentials plugin and with latest credentials plugin.

Submitter checklist

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• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed