RFR: 8325551: Remove unused obj_is_alive and block_start in Space [v2]

[Albert Mingkun Yang]

> Trivial removing dead code.

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17786/files
  - new: https://git.openjdk.org/jdk/pull/17786/files/3afe9290..e7962193

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17786&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17786&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 6 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