RFR: 8308181: Generational ZGC: Remove CLDG_lock from old gen root scanning

[Erik Österlund]

We already removed the CLDG_lock from young gen root scanning, after the CLDG was made concurrently walkable with [JDK-8307106](https://bugs.openjdk.org/browse/JDK-8307106). We should remove it from the old generation root scanning code as well.

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Commit messages:
 - Remove CLDG lock from old root scanning

Changes: https://git.openjdk.org/jdk/pull/14011/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14011&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308181
  Stats: 14 lines in 2 files changed: 0 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/14011.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14011/head:pull/14011

PR: https://git.openjdk.org/jdk/pull/14011