EOL Global-Mask-Classes

[basil]

Reverts commit a9fc20d. I discovered this unmitigated complexity while reading this code recently. I do not know if it was ever used, but it is completely unused today: I checked both Jenkins and CloudBees sources and nothing came up. I cannot imagine any good reason to use this "feature." Better to rip it out now while we still can before someone decides to start using it in the future and then we are obligated to support this unusual use case. The code comment mentions something about database plugins, but I checked them and none of them are using this feature, nor is anything else in the Jenkins ecosystem using it today.

As a bonus, ripping out this feature allows us to simplify the implementation of MaskingClassLoader to use immutable data structures, making it easier to reason about the concurrency implications of that class.

If accepted, I will also file subsequent PRs to purge any references to this "feature" from the HPI plugin and documentation.

Testing done

Started up Jenkins with java -jar jenkins.war with the jakarta-mail-api plugin installed, which exercises the (non-global) Mask-Classes code path and therefore the changed constructor. No errors.

Proposed changelog entries

N/A

Proposed upgrade guidelines

N/A

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[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!