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

[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.

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Commit messages:
 - 8254075: Shenandoah: Remove ShenandoahCodeRootsStyle diagnostic flag and related test

Changes: https://git.openjdk.java.net/jdk/pull/524/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=524&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254075
  Stats: 141 lines in 5 files changed: 0 ins; 129 del; 12 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