[JENKINS-74090] Remove unused checkUrl in TagAction/tagForm.jelly

[yaroslavafenkin]

https://issues.jenkins.io/browse/JENKINS-74090

Testing done

There's no corresponding doCheckTag method in TagAction.java or parent classes, so this is dead code.

I've also patched the latest version of the plugin (commenting out a few lines that were throwing exceptions) so that TagAction is added to my builds. I visited tagBuild page and no request is sent when I modify the field that has the checkUrl. Nor any CSP violations are logged in console or CSP report page.

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