RFR: 8263433: Shenandoah: Don't expect forwarded objects in set_concurrent_mark_in_progress()

[Zhengyu Gu]

Please review this trivial cleanup.

After removing piggyback reference update cycle, we don't expect to see forwarded objects during concurrent mark phase. 
While it is possible during full gc mark, but this flag is irrelevant.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8263433-fwd-mark

Changes: https://git.openjdk.java.net/jdk/pull/2941/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2941&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263433
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2941.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2941/head:pull/2941

PR: https://git.openjdk.java.net/jdk/pull/2941