RFR: JDK-8298111: Cleanups after UseMallocOnly removal

[Thomas Stuefe]

On Mon, 5 Dec 2022 16:57:29 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Trivial patch to remove unused prototypes from Arena.

Thank you, Coleen.

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PR: https://git.openjdk.org/jdk/pull/11516