RFR: 8331050: Serial: Remove unused _saved_mark_word in DefNewGeneration and TenuredGeneration [v2]

[Albert Mingkun Yang]

> Trivial removing dead code.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18931/files
  - new: https://git.openjdk.org/jdk/pull/18931/files/4553036b..5580bab9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18931&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18931&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 3 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