RFR: 8324636: Serial: Remove Generation::block_is_obj

[Albert Mingkun Yang]

On Wed, 24 Jan 2024 21:01:34 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

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

The doc is in `SerialHeap::block_is_obj`, the actual public API.

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PR Comment: https://git.openjdk.org/jdk/pull/17555#issuecomment-1908918876