RFR: 8331050: Serial: Remove unused TenuredGeneration::_saved_mark_word

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18931/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18931&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331050
  Stats: 2 lines in 1 file changed: 0 ins; 2 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18931.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18931/head:pull/18931

PR: https://git.openjdk.org/jdk/pull/18931