[JENKINS-74894] Remove workaround for JENKINS-19124

[yaroslavafenkin]

https://issues.jenkins.io/browse/JENKINS-74894

Testing done

Went to JENKINS/manage/cloud/create, selected vSphere cloud. No CSP violations were detected by the CSP plugin. Inspecting the credentials select element revealed there's no inline onchange handler. Everything behaves the same way after onchange got removed.

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