RFR: 8270082: Remove unnecessary gc_timer null check in ReferenceProcessorPhaseTimes

[Albert Mingkun Yang]

Simple change of removing a null check.

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Commit messages:
 - rp-timer

Changes: https://git.openjdk.java.net/jdk/pull/4720/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4720&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270082
  Stats: 8 lines in 1 file changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4720.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4720/head:pull/4720

PR: https://git.openjdk.java.net/jdk/pull/4720