RFR: 8318507: G1: Improve remset clearing for humongous candidates

[Thomas Schatzl]

Hi all,

  please review this small cleanup that merges two calls to `HeapRegionRemSet::clear()/HeapRegionRemSet::set_state_complete()` to a single call to `clear()` with appropriate parameters.

Also remove now obsolete asserts that are done in `clear()` already; actually the second one is wrong, there may be code roots attached to the region.

Testing: gha, tier1-5 almost done

Thanks,
  Thomas

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Commit messages:
 - Remove obsolete/incorrect asserts
 - initial version

Changes: https://git.openjdk.org/jdk/pull/16270/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16270&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318507
  Stats: 10 lines in 1 file changed: 0 ins; 7 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16270.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16270/head:pull/16270

PR: https://git.openjdk.org/jdk/pull/16270