RFR: 8326209: G1: Remove G1ConcurrentMark::_total_cleanup_time

[Albert Mingkun Yang]

Simple refactoring to remove redundant field, and change the time unit.

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Commit messages:
 - g1-cleanup-time

Changes: https://git.openjdk.org/jdk/pull/17918/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17918&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326209
  Stats: 6 lines in 2 files changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17918.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17918/head:pull/17918

PR: https://git.openjdk.org/jdk/pull/17918