Remove duplicate Jenkins push trigger

[JP0P]

Summary

Remove the redundant githubPush() trigger from the multibranch Jenkins pipeline.

Why

GitHub Branch Source already schedules builds for branch pushes. Keeping githubPush() enabled adds a second scheduling path for the same webhook, which can build the same application revision twice and create duplicate downstream artifacts and tests.

This has been observed in production: builds #160 and #161 both built commit 7795512. During validation, Jenkins also attempted to queue a second testMaestro build while #166 was already running.

Risk

Low. The final diff only removes three lines of Jenkins trigger configuration.

• GitHub Branch Source continues handling push builds.
• The periodic multibranch scan remains available for missed webhook events.
• Manual builds and disableConcurrentBuilds() remain unchanged.
• No application, build, artifact, test, deployment, or retry logic changes.

Both trigger mechanisms use Jenkins' /github-webhook/ endpoint, so githubPush() is not an independent delivery fallback. Any duplicate or missing webhook-delivery investigation is separate from removing this redundant scheduling path.

Validation

• After the updated Jenkinsfile ran, the child job no longer persisted GitHubPushTrigger.
• The one duplicate seen during the transition was expected because the child initially processed that push with its previously persisted trigger configuration.
• The current testMaestro job configuration contains no generic GitHub push trigger.
• CI and security checks pass.

The transition behavior does not recur after a child has loaded the updated Jenkinsfile.