RFR: 8253493: Shenandoah: Remove ShenandoahSerialRoot definition

[Zhengyu Gu]

JDK-8247281 removed ShenandoahSerialRoot's implementation and uses, but not its definition.

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Commit messages:
 - 8253493: Shenandoah: Remove ShenandoahSerialRoot definition

Changes: https://git.openjdk.java.net/jdk/pull/307/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=307&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253493
  Stats: 15 lines in 1 file changed: 0 ins; 15 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/307.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/307/head:pull/307

PR: https://git.openjdk.java.net/jdk/pull/307