RFR: 8343742: Test TestShenandoah.java fails Could not find leaking object

[SendaoYan]

Hi all,
 Test `jdk/jfr/event/oldobject/TestShenandoah.java` run timed out after [JDK-8279016](https://bugs.openjdk.org/browse/JDK-8279016), because of infinite loop of `Could not find leaking object, retrying...`
In [JDK-8279016](https://bugs.openjdk.org/browse/JDK-8279016), this test has been Problemlisted by [ProblemList-shenandoah.txt
](https://github.com/openjdk/jdk/blame/master/test/jdk/ProblemList-shenandoah.txt#L52), because of the LeakProfiler tests not suitable for run with `-XX:+UseShenandoahGC` after [JDK-8279016](https://bugs.openjdk.org/browse/JDK-8279016).
But in the VM option `-XX:+UseShenandoahGC` set inside the test tag cause the Problemlist mechanism will not take effect. The problemlist only take effect when the VM option set from command line such as `make TEST JTREG="JAVA_OPTIONS=-XX:+UseShenandoahGC"`.
So in this PR delete the VM option `-XX:+UseShenandoahGC` to make test run passed. The change has been verified locally, include:

- [x] linux x64 release build
- [x] linux x64 fastdebug build
- [x] linux aarch64 release build
- [x] linux aarch64 fastdebug build

Test-fix only, no risk.

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Commit messages:
 - 8343742: Test TestShenandoah.java fails Could not find leaking object

Changes: https://git.openjdk.org/jdk/pull/21951/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21951&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343742
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21951.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21951/head:pull/21951

PR: https://git.openjdk.org/jdk/pull/21951