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

[Aleksey Shipilev]

On Tue, 6 Oct 2020 14:39:32 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Shenandoah introduced fast code roots iteration (ShenandoahCodeRootsStyle = 2), also kept 2 additional iteration
>> mechanism (ShenandoahCodeRootsStyle = 0 and 1), mainly for verification purpose.
>> Today, Shenandoah normally iterates code roots concurrently using style 2, while style 0 and 1 require safepoints, that
>> makes style 0 and 1 verification less useful. Besides, Shenandoah has much improved verifier, more suitable for the
>> purpose.  Let's remove this diagnostic flag and related test.
>> 
>> Test:
>> 
>> - [x] hotspot_gc_shenandoah
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update

Marked as reviewed by shade (Reviewer).

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