RFR: 8290374: Shenandoah: Remove inaccurate comment on SBS::load_reference_barrier()

[Zhengyu Gu]

Please review this trivial patch to remove inaccurate comment.

Even with separate mark-compact marking phase, we still need `is_evacuation_in_progress()` check to deal with the scenario that Full GC was upgraded from concurrent/degenerated GC.

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Commit messages:
 - 8290374: Shenandoah: Remove inaccurate comment on SBS::load_reference_barrier()

Changes: https://git.openjdk.org/jdk/pull/9522/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9522&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290374
  Stats: 7 lines in 1 file changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9522.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9522/head:pull/9522

PR: https://git.openjdk.org/jdk/pull/9522