RFR: 8254075: Shenandoah: Remove ShenandoahCodeRootsStyle diagnostic flag and related test

[Aleksey Shipilev]

On Tue, 6 Oct 2020 14:01:42 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahConcurrentRoots.cpp line 40:
>> 
>>> 38:
>>> 39: bool ShenandoahConcurrentRoots::can_do_concurrent_class_unloading() {
>>> 40:   return true;
>> 
>> Hold on, this used to check `ClassUnloading`, should it continue to check it?
>
> 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.

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