RFR: 8263861: Shenandoah: Remove unused member in ShenandoahGCStateResetter

[Zhengyu Gu]

Please review this trivial patch that removed unused member of ShenandoahGCStateResetter.

Test:
- [x] hotspot_gc_shenandoah with -XX:+ShenandoahVerify

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Commit messages:
 - JDK-8263861

Changes: https://git.openjdk.java.net/jdk/pull/3088/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3088&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263861
  Stats: 4 lines in 2 files changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3088.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3088/head:pull/3088

PR: https://git.openjdk.java.net/jdk/pull/3088