RFR: 8300862: Remove some G1 collection set remembered set debugging code

[Thomas Schatzl]

Hi all,

  please review this removal of dead (debugging) code. I do not think it is necessary to keep such code, and apart from that I personally never used it.

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8300862 initial version

Changes: https://git.openjdk.org/jdk/pull/12144/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12144&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300862
  Stats: 53 lines in 2 files changed: 0 ins; 53 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12144.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12144/head:pull/12144

PR: https://git.openjdk.org/jdk/pull/12144