RFR: 8371680: JVMTI: Remove unused VMOp type JvmtiPostObjectFree

[Axel Boldt-Christmas]

The VMOp type `JvmtiPostObjectFree` was introduced with [JDK-8212879](https://bugs.openjdk.org/browse/JDK-8212879) but was never used for any `VM_Operation`. The `VM_Operation` introduced in that patch `VM_JvmtiPostObjectFree` used the `Cleanup` VMOp type instead.

`VM_JvmtiPostObjectFree` was later removed completely with [JDK-8256811](https://bugs.openjdk.org/browse/JDK-8256811). So there is no need for this VMOp type.

I suggest a trivial removal of this unused symbol.

* Testing
  * GHA

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Commit messages:
 - 8371680: JVMTI: Remove unused VMOp type JvmtiPostObjectFree

Changes: https://git.openjdk.org/jdk/pull/28254/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28254&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371680
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28254.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28254/head:pull/28254

PR: https://git.openjdk.org/jdk/pull/28254