Fixed spotbugs PATH_TRAVERSAL_IN issue in FileBoolean

[StefanSpieker]

Fixed Spotbugs PATH_TRAVERSAL_IN issue in FileBoolean with the suggested fix from find-sec-bugs

For this specific occurrence, I could also remove the method and adjust the usages. At least, according to GitHub search, it seems only to be used within the deploy-plugin.

See JENKINS-36720.

Testing done

Ran Spotbugs locally to verify the fix. I didn't test the plugin, which uses this method.

Proposed changelog entries

N/A

Proposed upgrade guidelines

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1. The Jira issue, if it exists, is well-described.
2. The changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developers, depending on the change) and are in the imperative mood (see examples). Fill in the Proposed upgrade guidelines section only if there are breaking changes or changes that may require extra steps from users during upgrade.
3. There is automated testing or an explanation as to why this change has no tests.
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5. New deprecations are annotated with @Deprecated(since = "TODO") or @Deprecated(forRemoval = true, since = "TODO"), if applicable.
6. New or substantially changed JavaScript is not defined inline and does not call eval to ease future introduction of Content Security Policy (CSP) directives (see documentation).
7. For dependency updates, there are links to external changelogs and, if possible, full differentials.
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