RFR: 8316021: Serial: Remove unused Generation::post_compact

[Thomas Schatzl]

On Mon, 11 Sep 2023 15:06:51 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial removing dead code.

Lgtm and trivial.

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PR Review: https://git.openjdk.org/jdk/pull/15663#pullrequestreview-1624384953