RFR: 8371998: G1: Rename G1MergeHeapRootsTask::G1ClearBitmapClosure to be more distinct

[Thomas Schatzl]

Hi all,

  please review the renaming of `G1MergeHeapRootsTask::G1ClearBitmapClosure` to something more distinct and appropriate to its function.

This also helps distinguishing it to the G1ClearBitmapHRClosure from concurrent marking that just does that.

Testing: compilation, gha

Thanks,
  Thomas

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Commit messages:
 - 8371998

Changes: https://git.openjdk.org/jdk/pull/28424/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28424&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8371998
  Stats: 8 lines in 1 file changed: 1 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/28424.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28424/head:pull/28424

PR: https://git.openjdk.org/jdk/pull/28424