RFR: 8252695: Shenandoah: clean up Graal enabled/disabled filter in tests

[zhengyu123]

On Mon, 7 Sep 2020 11:54:38 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Drop `!vm.graal.enabled` since it is not required after JDK-8249000.

There are a few more tests, I believe, need to be fixed as well:

gcbasher/TestGCBasherWithShenandoah.java
gclocker/TestGCLockerWithShenandoah.java
gcold/TestGCOldWithShenandoah.java
compiler/c2/aarch64/TestVolatilesShenandoah.java

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PR: https://git.openjdk.java.net/jdk/pull/51