RFR: 8302975: Remove redundant mark verification during G1 Full GC

[Thomas Schatzl]

Hi all,

  please review this remove of some redundant verification code that also opens opportunities for further cleanup in PR#12260. In particular it removes verification of marks during g1 full gc marking: this is checked after marking already, and actually in `G1FullGCMarker::mark_and_push` anyway.

To make this change small, I did not move the `G1VerifyLiveClosure` somewhere private, and anyway it will be refactored in PR#12260 after this has been pushed again..

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - removal

Changes: https://git.openjdk.org/jdk/pull/12687/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12687&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302975
  Stats: 20 lines in 3 files changed: 0 ins; 20 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12687.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12687/head:pull/12687

PR: https://git.openjdk.org/jdk/pull/12687