RFR: JDK-8298111: Cleanups after UseMallocOnly removal

[Thomas Stuefe]

Trivial patch to remove unused prototypes from Arena.

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Commit messages:
 - remove unused prototypes

Changes: https://git.openjdk.org/jdk/pull/11516/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298111
  Stats: 5 lines in 1 file changed: 0 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11516.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11516/head:pull/11516

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