RFR: 8324636: Serial: Remove Generation::block_is_obj

[Albert Mingkun Yang]

Trivial moving a method from `Generation` to subclass.

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Commit messages:
 - s1-is-block

Changes: https://git.openjdk.org/jdk/pull/17555/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17555&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324636
  Stats: 32 lines in 5 files changed: 8 ins; 23 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17555.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17555/head:pull/17555

PR: https://git.openjdk.org/jdk/pull/17555