RFR: 8325248: Serial: Remove Generation::space_iterate

[Albert Mingkun Yang]

Trivial removing `space_iterate` and inlining `block_start` to both subclasses.

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Commit messages:
 - s1-block-start

Changes: https://git.openjdk.org/jdk/pull/17709/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325248
  Stats: 47 lines in 6 files changed: 3 ins; 34 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/17709.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17709/head:pull/17709

PR: https://git.openjdk.org/jdk/pull/17709