[lilliput-jdk17u:lilliput] RFR: 8310642: [Lilliput/JDK17] Remove unnecessary check in G1ParScanThreadState

[Roman Kennke]

G1ParScanThreadState::do_copy_to_survivor_space() has an unnecessary change against upstream:

 if (old_mark.is_marked()) {
    // Already forwarded by somebody else, return forwardee.
    return old->forwardee(old_mark);
  }

All callers of this method already do the same, so this only adds code and possible affects performance for no reason.

Testing:
 - [ ] hotspot_gc -UCOH
 - [ ] hotspot_gc +UCOH
 - [ ] tier1 -UCOH
 - [ ] tier1 +UCOH
 - [ ] tier2 -UCOH
 - [ ] tier2 +UCOH

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Commit messages:
 - 8310642: [Lilliput/JDK17] Remove unnecessary check in G1ParScanThreadState

Changes: https://git.openjdk.org/lilliput-jdk17u/pull/42/files
 Webrev: https://webrevs.openjdk.org/?repo=lilliput-jdk17u&pr=42&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310642
  Stats: 4 lines in 1 file changed: 0 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/lilliput-jdk17u/pull/42.diff
  Fetch: git fetch https://git.openjdk.org/lilliput-jdk17u.git pull/42/head:pull/42

PR: https://git.openjdk.org/lilliput-jdk17u/pull/42