RFR: 8325551: Remove unused obj_is_alive and block_start in Space

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17786/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17786&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325551
  Stats: 19 lines in 3 files changed: 0 ins; 19 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17786.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17786/head:pull/17786

PR: https://git.openjdk.org/jdk/pull/17786