RFR: 8331719: Cleanup asserts/code after introducing IsSTWGCActiveMark

[xiaotaonan]

Cleanup asserts/code after introducing IsSTWGCActiveMark

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Commit messages:
 - Cleanup asserts/code after introducing IsSTWGCActiveMark

Changes: https://git.openjdk.org/jdk/pull/19167/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19167&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331719
  Stats: 2 lines in 2 files changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19167.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19167/head:pull/19167

PR: https://git.openjdk.org/jdk/pull/19167