RFR: 8304804: Remove develop flag G1VerifyCTCleanup

[Thomas Schatzl]

Hi all,

  please review the removal of the develop `G1VerifyCTCleanup` option. This develop flag seems to be not worth keeping as regular verification (`VerifyAfterGC`) performs this one anyway, and if it's too heavyweight it is trivial to add again for debugging.

There also seems to be no particular reason compared to others why you would specifically need card table verification more than other.

Testing: local compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8304804

Changes: https://git.openjdk.org/jdk/pull/13166/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13166&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8304804
  Stats: 4 lines in 2 files changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13166.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13166/head:pull/13166

PR: https://git.openjdk.org/jdk/pull/13166