RFR: 8262885: Shenandoah: FullGC prologue does not need to save/restore heap has_forwarded_object flag

[Zhengyu Gu]

Another trivial cleanup. FullGC prologue code does not change heap's has_forwarded_objects flag, there is no point to save/restore it.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8262885

Changes: https://git.openjdk.java.net/jdk/pull/2797/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2797&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262885
  Stats: 8 lines in 1 file changed: 0 ins; 6 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2797.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2797/head:pull/2797

PR: https://git.openjdk.java.net/jdk/pull/2797