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

[Kim Barrett]

On Fri, 13 Jan 2023 17:07:19 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial change of removing dead code.

Looks good.

While studying this I noticed that ContiguousSpace::reset_mark_word might not be
useful anymore either.  Consider making that a followup.

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Marked as reviewed by kbarrett (Reviewer).

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