Pull up AbstractFolder.delete logic into AbstractItem

[jglick]

Refactoring of #2789 since jenkinsci/cloudbees-folder-plugin#95 copied a bunch of complex code. As seen in jenkinsci/cloudbees-folder-plugin@cdf7370 this complicates maintenance. Started to clean up in jenkinsci/cloudbees-folder-plugin#353 + jenkinsci/cloudbees-folder-plugin#355 but to really remove the duplication it is simplest to just handle child deletion directly in AbstractItem.

Should have no behavioral effect for now since AbstractFolder overrides this, and the added block only pays attention to TopLevelItem, so should not affect MavenModuleSet or MatrixProject. jenkinsci/cloudbees-folder-plugin#356 completes the cleanup.

Testing done

JobTest.interruptOnDelete should check that non-folders are unaffected. Added a downstream test for what it is worth: jenkinsci/cloudbees-folder-plugin#356 (comment)

Proposed changelog entries

• N/A

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[jglick]

For now, retaining original behavior which is to say not calling delete() on children of non-top-level item groups (e.g., MatrixConfiguration). As in jenkinsci/cloudbees-folder-plugin#356 (comment), the only practical difference is likely to be in whether ItemListener is notified of these.

[Vlatombe]

Looks like an opportunity to add missing constructor signatures to AbortException.

[jglick]

Perhaps. It is not normal to call AbortException.initCause, since the point of this class is that it suppresses a stack trace (including causes & suppressed). Whether the initCause was ever actually helpful in jenkinsci/cloudbees-folder-plugin#95, I am not sure; neither AbstractFolder.delete nor AbstractItem.performDelete throw this exception type, so it may be dead code.

[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!