RFR: 8324636: Serial: Remove Generation::block_is_obj

[Y. Srinivas Ramakrishna]

On Wed, 24 Jan 2024 14:29:13 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial moving a method from `Generation` to subclass.

Please ensure that the header file documentation isn't lost because of this change. Looks fine otherwise.

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PR Review: https://git.openjdk.org/jdk/pull/17555#pullrequestreview-1842366026