RFR: 8340395: GenShen: Remove unnecessary check on card barrier flag

[William Kemper]

Clean backport.

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Commit messages:
 - Backport e11a68f8168675ce3ddced82860b9b3d288707df

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/112/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=112&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340395
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/112.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/112/head:pull/112

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/112