RFR: 8254075: Shenandoah: Remove ShenandoahCodeRootsStyle diagnostic flag and related test [v2]

[Zhengyu Gu]

On Tue, 6 Oct 2020 14:04:03 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> No. I actually puzzled why checked it here, our concurrent roots scanner can handle both scenarios.
>> Maybe a leftover from concurrent roots to concurrent class unloading transition?
>
> It is maybe something that disables concurrent class unloading when `-ClassUnloading` is specified? Although it should
> have been handled in `shenandoahArguments.cpp`. Still, I'd prefer to leave the behavior exact, and then remove the
> `ClassUnloading` reference in a separate PR.

Make sense, updated.

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PR: https://git.openjdk.java.net/jdk/pull/524