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

[Zhengyu Gu]

> 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

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/524/files
  - new: https://git.openjdk.java.net/jdk/pull/524/files/834fed17..093a63a4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=524&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=524&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/524.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/524/head:pull/524

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