RFR: 8324207: Serial: Remove Space::set_saved_mark_word

[Albert Mingkun Yang]

Simple refactoring to remove redundant save-mark-word calls.

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Commit messages:
 - s1-set-saved-mark

Changes: https://git.openjdk.org/jdk/pull/17496/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17496&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324207
  Stats: 7 lines in 4 files changed: 0 ins; 6 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17496.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17496/head:pull/17496

PR: https://git.openjdk.org/jdk/pull/17496