RFR: 8296670: G1: Remove unused G1GCPhaseTimes::record_preserve_cm_referents_time_ms

[Albert Mingkun Yang]

Trivial change of removing effectively dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/11061/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11061&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296670
  Stats: 8 lines in 2 files changed: 0 ins; 8 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11061.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11061/head:pull/11061

PR: https://git.openjdk.org/jdk/pull/11061