RFR: 8369793: G1: Remove unused G1FreeIdSet

[Thomas Schatzl]

Hi all,

  please review this trivial removal of the unused `G1FreeIdSet` code. JEP 522 made it obsolete.

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8369793

Changes: https://git.openjdk.org/jdk/pull/27786/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27786&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369793
  Stats: 167 lines in 2 files changed: 0 ins; 167 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27786.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27786/head:pull/27786

PR: https://git.openjdk.org/jdk/pull/27786