RFR: 8326319: G1: Remove unused G1ConcurrentMark::_init_times

[Albert Mingkun Yang]

Simple removing an always-zero counter.

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Commit messages:
 - g1-init-times

Changes: https://git.openjdk.org/jdk/pull/17928/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17928&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8326319
  Stats: 4 lines in 2 files changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17928.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17928/head:pull/17928

PR: https://git.openjdk.org/jdk/pull/17928