RFR: 8300125: Serial: Remove unused Generation::reset_saved_marks

[Albert Mingkun Yang]

Trivial change of removing dead code.

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Commit messages:
 - remove-reset_saved_marks

Changes: https://git.openjdk.org/jdk/pull/11994/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11994&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300125
  Stats: 17 lines in 5 files changed: 0 ins; 15 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11994.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11994/head:pull/11994

PR: https://git.openjdk.org/jdk/pull/11994