Move testDoNotifyCommitWithInvalidApiToken and use JenkinsRule

[MarkEWaite]

Move testDoNotifyCommitWithInvalidApiToken and use JenkinsRule

The GitStepTest already includes logic that uses notifyCommit to start jobs and perform other tests of notifyCommit. Use that logic to replace the mocked testDoNotifyCommitWithInvalidApiToken with a test that uses web requests to a JenkinsRule based test.

Prepare for Jetty 12 EE 9 transition.

• [JENKINS-73247] Adapt Git plugin for Jetty 12 (EE 9) #1589

Also removes unused GitSCMTest methods that were detected while reading the code.

Testing done

Confirmed that tests pass on my Linux computer with Java 21.

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