RFR: 8364760: G1: Remove obsolete code in G1MergeCardSetClosure

[Thomas Schatzl]

Hi all,

  please review this removal of some obsolete code.

Before JDK-8343782 G1 iterated remsets for humongous eager reclaim candidates by region, as there has been a 1:1 mapping between regions and (card-based) remembered sets.

With that change, all remembered set iteration should go via the group remsets (`G1CSetCandidateGroup`), and the code does, except for humongous eager reclaim candidates which handle their group remsets on the side (which is a separate issue).

This obsoleted some code in `G1MergeCardSetClosure` - it does not need to be a `G1HeapRegionClosure` any more, and for humongous regions we can directly do the iteration via the group rem set attached to the humongous starts region. I.e. inline that `merge_card_set_for_region()` method.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - 8364760

Changes: https://git.openjdk.org/jdk/pull/26645/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26645&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364760
  Stats: 38 lines in 1 file changed: 0 ins; 35 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/26645.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26645/head:pull/26645

PR: https://git.openjdk.org/jdk/pull/26645