RFR: 8325941: Remove unused Space::block_size

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17869/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17869&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325941
  Stats: 26 lines in 2 files changed: 0 ins; 26 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17869.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17869/head:pull/17869

PR: https://git.openjdk.org/jdk/pull/17869