RFR: 8277825: Remove unused ReferenceProcessorPhaseTimes::_sub_phases_total_time_ms

[Albert Mingkun Yang]

Simple change of removing effectively dead code.

Test: build

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Commit messages:
 - remove_unused

Changes: https://git.openjdk.java.net/jdk/pull/6557/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6557&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277825
  Stats: 17 lines in 2 files changed: 0 ins; 17 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6557.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6557/head:pull/6557

PR: https://git.openjdk.java.net/jdk/pull/6557